Fixed mention of minimum PHP version for v5.x

Minimum PHP version for Symfony 5.x is 7.2.5, but documentation doesn't reflect this change and keep same PHP version as 3.x.

Author: FlyingDR

contributing/code/pull_requests.rst

@@ -25,7 +25,7 @@ Before working on Symfony, setup a friendly environment with the following
 software:
 
 * Git;
-* PHP version 5.5.9 or above.
+* PHP version 7.2.5 or above.
 
 Configure Git
 ~~~~~~~~~~~~~