Convert readthedocs links for their .org -> .io migration for hosted projects (#413)

As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:

> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

docs/index.rst

@@ -9,7 +9,7 @@ For a more high level client library with more limited scope, have a look at
 `elasticsearch-dsl`_ - it is a more pythonic library sitting on top of
 ``elasticsearch-py``.
 
-.. _elasticsearch-dsl: http://elasticsearch-dsl.rtfd.org/
+.. _elasticsearch-dsl: https://elasticsearch-dsl.readthedocs.io/
 
 Compatibility
 -------------
@@ -78,7 +78,7 @@ Python. We have created some :ref:`helpers` to help with this issue as well as
 a more high level library (`elasticsearch-dsl`_) on top of this one to provide
 a more convenient way of working with Elasticsearch.
 
-.. _elasticsearch-dsl: http://elasticsearch-dsl.rtfd.org/
+.. _elasticsearch-dsl: https://elasticsearch-dsl.readthedocs.io/
 
 Persistent Connections
 ~~~~~~~~~~~~~~~~~~~~~~