Add examples using alternate capture/replace syntax

Author: MartinElvar

lib/elixir/lib/regex.ex

@@ -456,6 +456,9 @@ defmodule Regex do
       iex> Regex.replace(~r/a(b|d)c/, "abcadc", "[\\1]")
       "[b][d]"
 
+      iex> Regex.replace(~r/\.(\d)$/, "500.5", ".\\g{1}0")
+      "500.50"
+
       iex> Regex.replace(~r/a(b|d)c/, "abcadc", fn _, x -> "[#{x}]" end)
       "[b][d]"
 

lib/elixir/lib/string.ex

@@ -703,7 +703,7 @@ defmodule String do
   `\g{N}` in the `replacement` string to access a specific capture in the
   regex:
 
-      iex> String.replace("a,b,c", ~r/,(.)/, ",\\1\\1")
+      iex> String.replace("a,b,c", ~r/,(.)/, ",\\1\\g{1}")
       "a,bb,cc"
 
   Notice we had to escape the escape character `\`. By giving `\0`,