Fixes usage description of update_checkout.py

[valeriyvan]

/swift/utils/update-checkout --help outputs

usage: update-checkout [-h] [--clone] [--clone-with-ssh] [--skip-history]
                       [--skip-repository DIRECTORY] [--scheme BRANCH-SCHEME]
                       [--reset-to-remote] [--clean] [--config CONFIG]
                       [--github-comment GITHUB-COMMENT] [--dump-hashes]
                       [--dump-hashes-config BRANCH-SCHEME-NAME]
                       [--tag TAG-NAME] [--match-timestamp] [-j N_PROCESSES]
                       [--source-root SOURCE_ROOT]

repositories.

By default, updates your checkouts of Swift, SourceKit, LLDB, and SwiftPM.

optional arguments:
  -h, --help            show this help message and exit
  --clone               Obtain Sources for Swift and Related Projects
  --clone-with-ssh      Obtain Sources for Swift and Related Projects via SSH
  --skip-history        Skip histories when obtaining sources
  --skip-repository DIRECTORY
                        Skip the specified repository
  --scheme BRANCH-SCHEME
                        Use branches from the specified branch-scheme. A
                        "branch-scheme" is a list of (repo, branch) pairs.
  --reset-to-remote     Reset each branch to the remote state.
  --clean               Clean unrelated files from each repository.
  --config CONFIG       Configuration file to use
  --github-comment GITHUB-COMMENT
                        Check out related pull requests referenced in the
                        given free-form GitHub-style comment.
  --dump-hashes         Dump the git hashes of all repositories being tracked
  --dump-hashes-config BRANCH-SCHEME-NAME
                        Dump the git hashes of all repositories packaged into
                        update-checkout-config.json
  --tag TAG-NAME        Check out each repository to the specified tag.
  --match-timestamp     Check out adjacent repositories to match timestamp of
                        current swift checkout.
  -j N_PROCESSES, --jobs N_PROCESSES
                        Number of threads to run at once
  --source-root SOURCE_ROOT
                        The root directory to checkout repositories

Fixes it to be:

usage: update-checkout [-h] [--clone] [--clone-with-ssh] [--skip-history]
                       [--skip-repository DIRECTORY] [--scheme BRANCH-SCHEME]
                       [--reset-to-remote] [--clean] [--config CONFIG]
                       [--github-comment GITHUB-COMMENT] [--dump-hashes]
                       [--dump-hashes-config BRANCH-SCHEME-NAME]
                       [--tag TAG-NAME] [--match-timestamp] [-j N_PROCESSES]
                       [--source-root SOURCE_ROOT]

By default, updates your checkouts of Swift, SourceKit, LLDB, and SwiftPM
repositories.

optional arguments:
  -h, --help            show this help message and exit
  --clone               Obtain Sources for Swift and Related Projects
  --clone-with-ssh      Obtain Sources for Swift and Related Projects via SSH
  --skip-history        Skip histories when obtaining sources
  --skip-repository DIRECTORY
                        Skip the specified repository
  --scheme BRANCH-SCHEME
                        Use branches from the specified branch-scheme. A
                        "branch-scheme" is a list of (repo, branch) pairs.
  --reset-to-remote     Reset each branch to the remote state.
  --clean               Clean unrelated files from each repository.
  --config CONFIG       Configuration file to use
  --github-comment GITHUB-COMMENT
                        Check out related pull requests referenced in the
                        given free-form GitHub-style comment.
  --dump-hashes         Dump the git hashes of all repositories being tracked
  --dump-hashes-config BRANCH-SCHEME-NAME
                        Dump the git hashes of all repositories packaged into
                        update-checkout-config.json
  --tag TAG-NAME        Check out each repository to the specified tag.
  --match-timestamp     Check out adjacent repositories to match timestamp of
                        current swift checkout.
  -j N_PROCESSES, --jobs N_PROCESSES
                        Number of threads to run at once
  --source-root SOURCE_ROOT
                        The root directory to checkout repositories

Resolves SR-NNNN.

[shahmishal]

@swift-ci Python lint

[shahmishal]

@swift-ci smoke test and merge