CONTRIBUTING.md missing word in commit message formatting description

[rewolf]

Format commit messages using 55 characters for the subject line, 72 lines for the description, followed by the issue fixed, e.g. Fixes #22276.

"72 lines" should read 72-character lines to explain that lines in the description should not exceed 72 characters in width.

[snicoll]

@rewolf thank you for the report. FTR, there is no need to create both an issue and a PR as one supersedes the other.

Closing in favour of PR #22891