Update http_cache.rst

Replaced "two" by "three" on line 313.

Author: askeita

http_cache.rst

@@ -310,7 +310,7 @@ Safe Methods: Only caching GET or HEAD requests
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 HTTP caching only works for "safe" HTTP methods (like GET and HEAD). This means
-two things:
+three things:
 
 * Don't try to cache PUT or DELETE requests. It won't work and with good reason.
   These methods are meant to be used when mutating the state of your application