Revert "Fix bug in validators documentation (encode#8779)" (encode#8811)

This reverts commit 614bd87.

Author: auvipy

docs/api-guide/validators.md

@@ -271,7 +271,6 @@ A validator may be any callable that raises a `serializers.ValidationError` on f
     def even_number(value):
         if value % 2 != 0:
             raise serializers.ValidationError('This field must be an even number.')
-        return value
 
 #### Field-level validation