fix quickstart.rst (#276)

Author: shauryashahi

docs/source/quickstart.rst

@@ -52,7 +52,7 @@ create a subclass of ``django_elasticsearch_dsl.Document``,
 create a ``class Index`` inside the ``Document`` class
 to define your Elasticsearch indices, names, settings etc
 and at last register the class using ``registry.register_document`` decorator.
-It required to defined ``Document`` class in  ``documents.py`` in your app directory.
+It is required to define ``Document`` class in  ``documents.py`` in your app directory.
 
 .. code-block:: python