Fixes typo in exception message

Author: PhilCoggins

lib/ice_cube/rule.rb

@@ -29,7 +29,7 @@ def hash
     end
 
     def to_ical
-      raise MethodNotImplemented, "Expected to be overrridden by subclasses"
+      raise MethodNotImplemented, "Expected to be overridden by subclasses"
     end
 
     # Convert from ical string and create a rule