Convert readthedocs links for their .org -> .io migration for hosted projects

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.

Test Plan: Manually visited all the links I’ve modified.

README.rst

@@ -16,7 +16,7 @@ Easy way to use `bower <http://bower.io/>`_ with your `Django <https://www.djang
 
 Bower is a package manager for the web. It offers a generic, unopinionated solution to the problem of front-end package management, while exposing the package dependency model via an API that can be consumed by a more opinionated build stack. There are no system wide dependencies, no dependencies are shared between different apps, and the dependency tree is flat.
 
-Read full documentation on `read-the-docs <https://django-bower.readthedocs.org/en/latest/>`_.
+Read full documentation on `read-the-docs <https://django-bower.readthedocs.io/en/latest/>`_.
 
 Installation
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