minor #14639 Update http_cache.rst (askeita)

This PR was submitted for the 5.2 branch but it was merged into the 4.4 branch instead.

Discussion
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Update http_cache.rst

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

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2537437 Update http_cache.rst

Author: javiereguiluz

http_cache.rst

@@ -315,7 +315,7 @@ 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