Correct note on deprecation

Ensure index is not an alias for create index

Author: asya999

source/reference/method/db.collection.ensureIndex.txt

@@ -20,7 +20,7 @@ Definition
 
 
    .. deprecated:: 3.0.0
-      :method:`db.collection.ensureIndex()` is now an alias for
+      :method:`db.collection.ensureIndex()` has been replaced by 
       :method:`db.collection.createIndex()`.
 
    Creates an index on the specified field if the index does not