lints/doc: fix typo in docs of too_long_first_doc_paragraph

[vobst]

Fixing a small typo in the description of the too_long_first_doc_paragraph lint.

changelog: [too_long_first_doc_paragraph]: Fix a typo in the lint's documentation.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @llogiq (or someone else) some time within the next two weeks.

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[bors]

☔ The latest upstream changes (presumably #13586) made this pull request unmergeable. Please resolve the merge conflicts.

[llogiq]

r=me after a rebase.

[vobst]

r=llogiq In fact I realized that #13586 makes this one obsolete. So I'd just suggest closing this one. Cheers