Fix typo in list_collections docstring (collectons -> collections) (#644)

(cherry picked from commit cfbc3a7)

Author: sharat87

pymongo/database.py

@@ -810,7 +810,7 @@ def _list_collections(self, sock_info, slave_okay, session,
         return cmd_cursor
 
     def list_collections(self, session=None, filter=None, **kwargs):
-        """Get a cursor over the collectons of this database.
+        """Get a cursor over the collections of this database.
 
         :Parameters:
           - `session` (optional): a