Fix String.valid_codepoint? tests

Author: meh

lib/elixir/lib/string.ex

@@ -470,7 +470,7 @@ defmodule String do
       true
       iex> String.valid_codepoint?("ø")
       true
-      iex> String.valid_codepoint?("\xffff")
+      iex> String.valid_codepoint?(<<0xffff :: 16>>)
       false
       iex> String.valid_codepoint?("asdf")
       false

lib/elixir/test/elixir/string_test.exs

@@ -233,7 +233,7 @@ defmodule StringTest do
     assert String.valid_codepoint?("ø")
     assert String.valid_codepoint?("あ")
 
-    refute String.valid_codepoint?("\xffff")
+    refute String.valid_codepoint?(<<0xffff :: 16>>)
     refute String.valid_codepoint?("ab")
   end