minor #16144 [HTTP Cache] Fix grammar issue (sebpacz)

This PR was merged into the 4.4 branch.

Discussion
----------

[HTTP Cache] Fix grammar issue

<!--

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
-------

c51c80d [HTTP Cache] Fix grammar issue

Author: javiereguiluz

http_cache/ssi.rst

@@ -126,7 +126,7 @@ The ``render_ssi`` twig helper will generate something like:
 
     <!--#include virtual="/_fragment?_hash=abcdef1234&_path=_controller=App\Controller\ProfileController::gdpr" -->
 
-``render_ssi`` ensures that SSI directive are generated only if the request
+``render_ssi`` ensures that SSI directive is generated only if the request
 has the header requirement like ``Surrogate-Capability: device="SSI/1.0"``
 (normally given by the web server).
 Otherwise it will embed directly the sub-response.