Improve disabled tag error output

lib/liquid/locales/en.yml

@@ -26,4 +26,4 @@
     argument:
       include: "Argument error in tag 'include' - Illegal template name"
     disabled:
-      tag: "usage has been disabled in this context."
+      tag: "usage is not allowed in this context"

lib/liquid/tags/render.rb

@@ -3,7 +3,7 @@
 module Liquid
   class Render < Tag
     SYNTAX = /(#{QuotedString})#{QuotedFragment}*/o
-    DISABLED_TAGS = %w(include)
+    DISABLED_TAGS = %w(include).freeze
 
     attr_reader :template_name_expr, :attributes