Typos in serializers documentation (encode#5652)

Fixes encode#5651.

Change `update()` to `.update()` in serializers documentation to get a
consistency with `.create()`.

Author: hangpark

docs/api-guide/serializers.md

@@ -73,7 +73,7 @@ Deserialization is similar. First we parse a stream into Python native datatypes
 
 ## Saving instances
 
-If we want to be able to return complete object instances based on the validated data we need to implement one or both of the `.create()` and `update()` methods. For example:
+If we want to be able to return complete object instances based on the validated data we need to implement one or both of the `.create()` and `.update()` methods. For example:
 
     class CommentSerializer(serializers.Serializer):
         email = serializers.EmailField()
@@ -325,7 +325,7 @@ For updates you'll want to think carefully about how to handle updates to relati
 * Ignore the data and leave the instance as it is.
 * Raise a validation error.
 
-Here's an example for an `update()` method on our previous `UserSerializer` class.
+Here's an example for an `.update()` method on our previous `UserSerializer` class.
 
         def update(self, instance, validated_data):
             profile_data = validated_data.pop('profile')