Mark PHP function

Co-authored-by: Antoine Lamirault <[email protected]>

Author: tcoch

reference/constraints/Slug.rst

@@ -95,7 +95,7 @@ Options
 **type**: ``string`` default: ``/^[a-z0-9]+(?:-[a-z0-9]+)*$/``
 
 This option allows you to modify the regular expression pattern that the input will be matched against
-via the preg_match PHP function.
+via the :phpfunction:`preg_match` PHP function.
 
 If you need to use it, you might also want to take a look at the :doc:`Regex constraint <Regex>`.