minor #15548 #42229 update phpdoc to recommend createIndex over ensureIndex (kralos)

This PR was merged into the 4.4 branch.

Discussion
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#42229 update phpdoc to recommend createIndex over ensureIndex

See symfony/symfony#42229

Commits
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0f1a5ff #42229 update phpdoc to recommend createIndex over ensureIndex

Author: javiereguiluz

session/database.rst

@@ -532,7 +532,7 @@ performance. Run this from the `MongoDB shell`_:
 .. code-block:: javascript
 
     use session_db
-    db.session.ensureIndex( { "expires_at": 1 }, { expireAfterSeconds: 0 } )
+    db.session.createIndex( { "expires_at": 1 }, { expireAfterSeconds: 0 } )
 
 Configuring the Session Field Names
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~