Fix PHP version number in PHP54MigrationSet description

doc/ruleSets/PHP54Migration.rst

@@ -2,7 +2,7 @@
 Rule set ``@PHP54Migration``
 ============================
 
-Rules to improve code for PHP 5.6 compatibility.
+Rules to improve code for PHP 5.4 compatibility.
 
 Rules
 -----

src/RuleSet/Sets/PHP54MigrationSet.php

@@ -28,6 +28,6 @@ final class PHP54MigrationSet extends AbstractRuleSetDescription
 
     public function getDescription()
     {
-        return 'Rules to improve code for PHP 5.6 compatibility.';
+        return 'Rules to improve code for PHP 5.4 compatibility.';
     }
 }