docs: Fix typo in analyze-namespace option in the README

[parroty]

What's being changed?

Hi 👋 . This PR just intends to fix the example description for --namespace option. I believe the option name is singular, though it can take multiple items.

from execution

$ legitify analyze --org sample-org-1 --namespaces organization,member

 ___      _______  _______  ___   _______  ___   _______  __   __
|   |    |       ||       ||   | |       ||   | |       ||  | |  |
|   |    |    ___||    ___||   | |_     _||   | |    ___||  |_|  |
|   |    |   |___ |   | __ |   |   |   |  |   | |   |___ |       |
|   |___ |    ___||   ||  ||   |   |   |  |   | |    ___||_     _|
|       ||   |___ |   |_| ||   |   |   |  |   | |   |      |   |
|_______||_______||_______||___|   |___|  |___| |___|      |___|
By Legit Security

Error: unknown flag: --namespaces
2022/11/24 20:17:06 error executing command: unknown flag: --namespaces

from help

$ legitify analyze --help
 ___      _______  _______  ___   _______  ___   _______  __   __
|   |    |       ||       ||   | |       ||   | |       ||  | |  |
|   |    |    ___||    ___||   | |_     _||   | |    ___||  |_|  |
|   |    |   |___ |   | __ |   |   |   |  |   | |   |___ |       |
|   |___ |    ___||   ||  ||   |   |   |  |   | |    ___||_     _|
|       ||   |___ |   |_| ||   |   |   |  |   | |   |      |   |
|_______||_______||_______||___|   |___|  |___| |___|      |___|
By Legit Security

Analyze GitHub organizations associated with a PAT to find security issues

Usage:
  legitify analyze [flags]

Flags:
      --color string            when to use coloring [auto/always/none] (default "auto")
      --error-file string       error log path (default "error.log")
      --failed-only             Only show violated policied (do not show succeeded/skipped)
  -t, --github-token string     token to authenticate with github (required unless environment variable GITHUB_TOKEN is set)
  -h, --help                    help for analyze
  -n, --namespace strings       which namespace to run (default [organization,repository,member,actions])
      --org strings             specific organizations to collect
      --output-file string      output file, defaults to stdout
  -f, --output-format string    output format [json/human] (default "human")
      --output-scheme string    output scheme [group-by-resource/group-by-severity/flattened/group-by-namespace] (default "flattened")
  -p, --policies-path strings   directory containing opa policies
      --repo strings            specific repositories to collect (--repo owner/repo_name (e.g. ossf/scorecard)
      --scorecard string        Whether to run additional scorecard checks [no/yes/verbose] (default "no")

Is this PR related to an existing issue?

Check off the following:

• This PR follows the CONTRIBUTION.md guidelines
• I have self-reviewed my changes before submitting the PR

[noamd-legit]

@parroty You're right! Merging