minor #5740 Fix typo in PdoSessionHandler Documentation (tobemedia)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #5740).

Discussion
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Fix typo in PdoSessionHandler Documentation

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

fixed typo on xml service configuration

Commits
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d735cde Update pdo_session_storage.rst

Author: xabbuh

cookbook/doctrine/pdo_session_storage.rst

@@ -170,7 +170,7 @@ of your project's data, you can use the connection settings from the
     .. code-block:: xml
 
         <service id="session.handler.pdo" class="Symfony\Component\HttpFoundation\Session\Storage\Handler\PdoSessionHandler" public="false">
-            <argument>mysql:host=%database_host%;port=%database_port%;dbname=%database_name%</agruement>
+            <argument>mysql:host=%database_host%;port=%database_port%;dbname=%database_name%</argument>
             <argument type="collection">
                 <argument key="db_username">%database_user%</argument>
                 <argument key="db_password">%database_password%</argument>