fix typos of source comments at Enumeration model

git-svn-id: http://svn.redmine.org/redmine/trunk@13072 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/enumeration.rb

@@ -73,7 +73,7 @@ class Enumeration < ActiveRecord::Base
     self.objects_count != 0
   end
 
-  # Is this enumeration overiding a system level enumeration?
+  # Is this enumeration overriding a system level enumeration?
   def is_override?
     !self.parent.nil?
   end