Correct typo in error message

Author: chdastolfo

clever/models/event.py

@@ -146,7 +146,7 @@ def data(self, data):
         :type: object
         """
         if data is None:
-            raise ValueError("Invalid value for `type`, must not be `None`")
+            raise ValueError("Invalid value for `data`, must not be `None`")
 
         self._data = data