Remove duplicate line in formatter_callback doc (#13387)

Author: codeanpeace

lib/ex_unit/lib/ex_unit/formatter.ex

@@ -120,8 +120,6 @@ defmodule ExUnit.Formatter do
 
     * `:error_info` - Should format error information.
 
-    * `:error_info` - Should format error information.
-
     * `:test_module_info` - Should format test module information. The message returned
     when this key is passed precedes messages such as `"failure on setup_all callback [...]"`.