[Sass] Fix the callback example indentation.

lib/sass/callbacks.rb

@@ -6,21 +6,21 @@
 # and passing in a block that's run when the callback is activated.
 #
 # @example Define a callback
-#   class Munger
-#     extend Sass::Callbacks
-#     define_callback :string_munged
+# class Munger
+#   extend Sass::Callbacks
+#   define_callback :string_munged
 #
-#     def munge(str)
-#       res = str.gsub(/[a-z]/, '\1\1')
-#       run_string_munged str, res
-#       res
-#     end
+#   def munge(str)
+#     res = str.gsub(/[a-z]/, '\1\1')
+#     run_string_munged str, res
+#     res
 #   end
+# end
 #
 # @example Use a callback
-#   m = Munger.new
-#   m.on_string_munged {|str, res| puts "#{str} was munged into #{res}!"}
-#   m.munge "bar" #=> bar was munged into bbaarr!
+# m = Munger.new
+# m.on_string_munged {|str, res| puts "#{str} was munged into #{res}!"}
+# m.munge "bar" #=> bar was munged into bbaarr!
 module Sass::Callbacks
   protected