minor symfony#7356 Fixed typo - changed it's to its (mohdshakir)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes symfony#7356).

Discussion
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Fixed typo - changed it's to its

Commits
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2f47419 Fixed typo - changed it's to its

Author: xabbuh

session/php_bridge.rst

@@ -11,7 +11,7 @@ If you're integrating the Symfony full-stack Framework into a legacy application
 that starts the session with ``session_start()``, you may still be able to
 use Symfony's session management by using the PHP Bridge session.
 
-If the application has it's own PHP save handler, you can specify null
+If the application has its own PHP save handler, you can specify null
 for the ``handler_id``:
 
 .. configuration-block::