Update http_cache.rst

Took me quite a while to figure this out; this simple tip might help others save that time :-)
Merge branch 'master' of github.com:alcohol/Silex

* 'master' of github.com:alcohol/Silex:
  Update http_cache.rst
  Update http_cache.rst
  Update http_cache.rst
  Update http_cache.rst
  Update http_cache.rst
  Update http_cache.rst