docs: replace "are" with "must be"

Author: kenjis

contributing/pull_request.md

@@ -4,7 +4,7 @@
 
 We expect the following in all Pull Requests (PRs).
 - PRs must be sent to the [appropriate branch](#branching)
-- All git commits are [GPG-signed](#signing)
+- All git commits must be [GPG-signed](#signing)
 - Must follow our [style guide](#php-style)
 - Be [commented](#comments) in the PHP source file
 - Be documented in the [user guide](#user-guide)