minor #16245 Correct spelling & grammar in 4.4 http_cache.rst (gnito-org)

This PR was merged into the 4.4 branch.

Discussion
----------

Correct spelling & grammar in 4.4 http_cache.rst

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/releases for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `5.x` for features of unreleased versions).

-->

Commits
-------

1f1285b Correct spelling & grammar in 4.4 http_cache.rst

Author: javiereguiluz

http_cache.rst

@@ -71,7 +71,7 @@ Symfony Reverse Proxy
 
 Symfony comes with a reverse proxy (i.e. gateway cache) written in PHP.
 :ref:`It's not a fully-featured reverse proxy cache like Varnish <http-cache-symfony-versus-varnish>`,
-but is a great way to start.
+but it is a great way to start.
 
 .. tip::