docs: make function name linkable

Author: kenjis

user_guide_src/source/general/helpers.rst

@@ -139,7 +139,7 @@ functions:
 
 .. literalinclude:: helpers/006.php
 
-The ``helper()`` function will scan through all PSR-4 namespaces defined in **app/Config/Autoload.php**
+The :php:func:`helper()` function will scan through all PSR-4 namespaces defined in **app/Config/Autoload.php**
 and load in ALL matching helpers of the same name. This allows any module's helpers
 to be loaded, as well as any helpers you've created specifically for this application. The load order
 is as follows: