Update multi_datastream.py

the key for EntityTypes should read "MultiDatastream"

Author: tmartini

frost_sta_client/dao/multi_datastream.py

@@ -23,4 +23,4 @@ def __init__(self, service):
         """
         A data access object for operations with the MultiDatastream entity
         """
-        base.BaseDao.__init__(self, service, EntityTypes["MultiDatastreams"])
+        base.BaseDao.__init__(self, service, EntityTypes["MultiDatastream"])