minor #7038 Remove a dead link to the old PR header (dunglas)

This PR was submitted for the 2.3 branch but it was merged into the 2.7 branch instead (closes #7038).

Discussion
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Remove a dead link to the old PR header

Commits
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0ffebaf Remove a dead link to the PR header

Author: xabbuh

.github/CONTRIBUTING.md

@@ -2,6 +2,4 @@ Contributing
 ------------
 
 We love contributors! For more information on how you can contribute to the
-Symfony documentation, please read [Contributing to the Documentation](https://symfony.com/doc/current/contributing/documentation/overview.html)
-and notice the [Pull Request Format](https://symfony.com/doc/current/contributing/documentation/overview.html#pull-request-format)
-that helps us merge your pull requests faster!
+Symfony documentation, please read [Contributing to the Documentation](https://symfony.com/doc/current/contributing/documentation/overview.html).