Rename some copy pasted variable.

Author: jhgg

epoxy/metaclasses/scalar.py

@@ -24,7 +24,7 @@ def __new__(mcs, name, bases, attrs):
         ))
 
     @staticmethod
-    def _register(mutation):
+    def _register(scalar):
         raise NotImplementedError('_register must be implemented in the sub-metaclass')
 
     @staticmethod