minor #15202 Corrected minor grammar mistake in the routing docs (settermjd)

This PR was submitted for the 5.x branch but it was merged into the 5.2 branch instead.

Discussion
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Corrected minor grammar mistake in the routing docs

I was reading through the routing documentation and noticed a tiny grammar mistake, which I thought I should fix. Nothing major.

Commits
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f68ec04 Corrected minor grammar mistake in the routing docs

Author: wouterj

routing.rst

@@ -2216,8 +2216,8 @@ Stateless Routes
     The ``stateless`` option was introduced in Symfony 5.1.
 
 Sometimes, when an HTTP response should be cached, it is important to ensure
-that can happen. However, whenever session is started during a request, Symfony
-turns the response into a private non-cacheable response.
+that can happen. However, whenever a session is started during a request,
+Symfony turns the response into a private non-cacheable response.
 
 For details, see :doc:`/http_cache`.