Fixed missing "fields" meta argument in docs (#8243)

* Fixed missing "fields" meta argument in docs, leading to an assertion error

* Update docs/api-guide/serializers.md

Co-authored-by: Tom Christie <[email protected]>

Co-authored-by: Tom Christie <[email protected]>

Author: Guilouf

docs/api-guide/serializers.md

@@ -524,6 +524,7 @@ You can add extra fields to a `ModelSerializer` or override the default fields b
 
         class Meta:
             model = Account
+            fields = ['url', 'groups']
 
 Extra fields can correspond to any property or callable on the model.