Change 'a' to 'an' for 'attribute' word [ci skip]

activerecord/lib/active_record/attributes.rb

@@ -14,7 +14,7 @@ module Attributes # :nodoc:
     end
 
     module ClassMethods # :nodoc:
-      # Defines or overrides a attribute on this model. This allows customization of
+      # Defines or overrides an attribute on this model. This allows customization of
       # Active Record's type casting behavior, as well as adding support for user defined
       # types.
       #