Fix typo in cargo-yank docs

[alexhrao]

This fixes a (very small) typo in the docs for cargo-yank. Specifically, since "unintentional SemVer breakages" is plural, the "an" that was at the beginning of that sentence was incorrect.

What does this PR try to resolve?

There wasn't an issue, just a very tiny typo

How to test and review this PR?

Hopefully this shouldn't need any testing... it's just a change to a markdown file

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• Owners of files modified in this PR: @ehuss, @epage, @weihanglo
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[epage]

You'll need to run:

cargo build-man
SNAPSHOTS=overwrite cargo test

and amend your commit with the result to pass CI

[alexhrao]

I'm not entirely sure what I did wrong here... I ran the following

cargo clean
cargo build-man
SNAPSHOTS=overwrite cargo test

And committed all the changed files. I see in some of the checks it mentions to use SNAPSHOTS=overwrite... Isn't that what the third line would have done? Any guidance on what I'm doing wrong here would be greatly appreciated!

[epage]

Unsure why it included all of that content but I would revert the unrelated snapshots

[alexhrao]

Okay, so specifically I should remove the changes to tests/testsuite/build_dir.rs and tests/testsuite/build_dir_legacy.rs?

[epage]

Thanks!