Fix String.at/1 documentation example

Author: yrashk

lib/elixir/lib/string.ex

@@ -447,7 +447,7 @@ defmodule String do
 
   ## Examples
 
-      String.at("elixir", 0) #=> "1"
+      String.at("elixir", 0) #=> "e"
       String.at("elixir", 1) #=> "l"
       String.at("elixir", 10) #=> nil
       String.at("elixir", -1) #=> "r"