Remove Py2/Py3 difference from Example (#4145)

`print` is available as a function on every supported version of Python (2.7+)

docs/api-guide/serializers.md

@@ -452,7 +452,7 @@ To do so, open the Django shell, using `python manage.py shell`, then import the
 
     >>> from myapp.serializers import AccountSerializer
     >>> serializer = AccountSerializer()
-    >>> print repr(serializer)  # Or `print(repr(serializer))` in Python 3.x.
+    >>> print(repr(serializer))
     AccountSerializer():
         id = IntegerField(label='ID', read_only=True)
         name = CharField(allow_blank=True, max_length=100, required=False)