Minor simplification

Author: javiereguiluz

reference/constraints/Url.rst

@@ -244,7 +244,7 @@ or a regular expression::
     // (e.g. 'https://', 'git+ssh://', 'file://', 'custom://')
     protocols: ['*']
 
-    // regular expressions compatible with PHP's preg_match() are also valid
+    // regular expressions are also valid
     protocols: ['https?', 'custom.*', 'my-app-.*']
 
 .. versionadded:: 7.4