chore(gazelle): clarify a particular failure message

[srabraham]

It was kind of ambiguous before about where the user should look to find instructions on how to resolve the issue.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

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[f0rmiga]

Looks good. Could you just run the update command in https://buildkite.com/bazel/rules-python-python/builds/3811#018535cd-f283-4110-940e-50bd1b9ae714?