docs: fix section title underline

Co-authored-by: Pooya Parsa Dadashi <pooya_parsa_dadashi@yahoo.com>

Author: kenjis

contributing/documentation.rst

@@ -137,7 +137,7 @@ To a Function
     :php:func:`dot_array_search()`
 
 To a Method
-=============
+===========
 
 ::