docs: remove stray directory metavar from cargo update --breaking option

[GuTS805]

What does this PR try to resolve?

The --breaking option in cargo update is documented as taking a directory argument:

--breaking directory
    Update spec to latest SemVer-breaking version.

This is incorrect — --breaking is a plain boolean flag with no argument. It's defined via the flag(...) helper in src/bin/cargo/commands/update.rs and checked with args.flag("breaking"), the same pattern used by --recursive/--dry-run, which correctly show no metavar. Running cargo update --help confirms this:

-b, --breaking  Update [SPEC] to latest SemVer-breaking version (unstable)

The stray _directory_ appears to be a copy-paste artifact from other flags that legitimately take a directory value (e.g. --artifact-dir, --target-dir), most likely introduced when --breaking was added in #13979.

This PR removes the incorrect metavar from the doc/man source (doc/man/cargo-update.md) and regenerates the derived outputs (doc/man/generated_txt/cargo-update.txt, etc/man/cargo-update.1, doc/book/src/commands/cargo-update.md) via cargo build-man, per the documentation contributor guide.

How to test and review this PR?

Compare the rendered --breaking entry before/after in doc/man/generated_txt/cargo-update.txt or doc/book/src/commands/cargo-update.md — it should no longer show directory after --breaking. This is a docs-only change; no behavior is affected.

Fixes #17318

[rustbot]

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @epage, @weihanglo
• @epage, @weihanglo expanded to epage, weihanglo
• Random selection from epage, weihanglo

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[weihanglo]

Thanks.

View changes since this review

[GuTS805]

Thanks for the review