Update extending.rst, fix typo

completele -> completely

Author: Simon Tite

docs/extending.rst

@@ -56,7 +56,7 @@ Hook
 ----
 
 Hooks are used for easy and quick manipulation with generated ElementTree elements. Hooks are called for each newly created element.
-Using hooks we are able to slightly modify or completele rewrite content generated by serializers.
+Using hooks we are able to slightly modify or completely rewrite content generated by serializers.
 
 
 Example