Mark the XML lexer class as private

I was supposed to mark this as private for 1.0 but completely forgot.

lib/oga/xml/lexer.rb

@@ -37,6 +37,8 @@ module Oga
     # @!attribute [r] html
     #  @return [TrueClass|FalseClass]
     #
+    # @private
+    #
     class Lexer
       attr_reader :html