bug #5589 [Cookbook][Session] fix default expiry field name (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Cookbook][Session] fix default expiry field name

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

As @stevenmusumeche noted in #5478 (comment), the [default expiry field name is expires_at](https://github.com/symfony/symfony/blob/2.8/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php#L86).

Commits
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1e1a129 [Cookbook][Session] fix default expiry field name

cookbook/doctrine/mongodb_session_storage.rst

@@ -165,7 +165,7 @@ From the `MongoDB shell`_:
 .. code-block:: sql
 
     use session_db
-    db.session.ensureIndex( { "expireAt": 1 }, { expireAfterSeconds: 0 } )
+    db.session.ensureIndex( { "expires_at": 1 }, { expireAfterSeconds: 0 } )
 
 .. _installed and configured a MongoDB server: http://docs.mongodb.org/manual/installation/
 .. _MongoDB shell: http://docs.mongodb.org/v2.2/tutorial/getting-started-with-the-mongo-shell/