minor #15346 [RateLimiter] [Rate Limiter] Added missing opening single quotes (artyuum)

This PR was merged into the 5.2 branch.

Discussion
----------

[RateLimiter] [Rate Limiter] Added missing opening single quotes

Added the missing opening single quotes in the PHP config example

<!--

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
-------

87dbab9 [Rate Limiter] Added missing opening single quotes

Author: javiereguiluz

rate_limiter.rst

@@ -410,7 +410,7 @@ Use the ``cache_pool`` option to override the cache used by a specific limiter
 
         // config/packages/rate_limiter.php
         $container->loadFromExtension('framework', [
-            rate_limiter' => [
+            'rate_limiter' => [
                 'anonymous_api' => [
                     // ...