Remind people to have 'Fixes #XXXX' in the commit message

It's of course also possible to just add them to the PR description,
but having these lines in the commit messages provide better
automation.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3104)

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,7 +3,9 @@ Thank you for your pull request. Please review these requirements:
 
 Contributors guide: https://github.com/openssl/openssl/blob/master/CONTRIBUTING
 
-Other than that, provide a description above this comment if there isn't one already, and if it fixes a github issue, add 'Fixes #XXXX' (without quotes) as well.
+Other than that, provide a description above this comment if there isn't one already
+
+If this fixes a github issue, make sure to have a line saying 'Fixes #XXXX' (without quotes) in the commit message.
 -->
 
 ##### Checklist