.mailmap: normalize emails for Jeff King

I never meant anything special by using my @github.com
address; it is merely a mistake that it has sometimes bled
through to patches.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.mailmap

@@ -31,6 +31,7 @@ Horst H. von Brand <vonbrand@inf.utfsm.cl>
 İsmail Dönmez <ismail@pardus.org.tr>
 Jakub Narębski <jnareb@gmail.com>
 Jay Soffian <jaysoffian+git@gmail.com>
+Jeff King <peff@peff.net> <peff@github.com>
 Joachim Berdal Haga <cjhaga@fys.uio.no>
 Johannes Sixt <j6t@kdbg.org> <johannes.sixt@telecom.at>
 Johannes Sixt <j6t@kdbg.org> <j.sixt@viscovery.net>