fix: typo in README_FR (#14179)

[VinceOPS]

Summary

Fixes #14179

• Line break in the middle of a title.
• Invalid use of : (in French, we must add a whitespace before).

Screenshots

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[crazywoola]

Thanks man :)

I always want to learn some French. I took a lesson from Duolingo, but I can only remember croissant.