Import parser added to docs in API guide

Added missing import statements in code snippet of API guide.

Closes-Issue: #3727

docs/api-guide/parsers.md

@@ -52,6 +52,9 @@ using the `APIView` class based views.
 
 Or, if you're using the `@api_view` decorator with function based views.
     
+    from rest_framework.decorators import api_view
+    from rest_framework.decorators import parser_classes
+
     @api_view(['POST'])
     @parser_classes((JSONParser,))
     def example_view(request, format=None):