Fix bug in validators documentation (#8779)

Function validators seem to have to return their value (which will become part of `validated_data`) as part of their signature.

docs/api-guide/validators.md

@@ -271,6 +271,7 @@ 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