Fix typo (grammatical error in exception message).

Author: acolomb

canopen/sdo/client.py

@@ -571,7 +571,7 @@ def _retransmit(self):
                 return response
         self._error = True
         self.sdo_client.abort(0x0504_0000)
-        raise SdoCommunicationError("Some data were lost and could not be retransmitted")
+        raise SdoCommunicationError("Some data was lost and could not be retransmitted")
 
     def _ack_block(self):
         request = bytearray(8)