Fix typo in docs/source/topics/known-issues.rst

Author: timgraham

docs/source/topics/known-issues.rst

@@ -101,4 +101,4 @@ Caching
 
 Secondly, you must use the :class:`django_mongodb_backend.cache.MongoDBCache`
 backend rather than Django's built-in database cache backend,
-``django.core.cache.backends.db.DatabaseCache``).
+``django.core.cache.backends.db.DatabaseCache``.