[1.6.x] Fixed ReST typo in topics/class-based-views/mixins.txt

Backport of 0b35a2c from master

docs/topics/class-based-views/mixins.txt

@@ -345,7 +345,7 @@ The ``paginate_by`` is deliberately small in the example so you don't
 have to create lots of books to see the pagination working! Here's the
 template you'd want to use:
 
-.. code-block: html+django
+.. code-block:: html+django
 
     {% extends "base.html" %}