mailmap: change primary address for Christian Couder

mailmap: change primary address for Christian Couder

The `chriscool@tuxfamily.org` address is an old one that I don't use
anymore, while `christian.couder@gmail.com` is the address I have been
sending patches from for a long time.

Let's swap the two addresses in the existing entry, so that the Gmail
address becomes the primary one and the old tuxfamily.org address is
mapped to it. This way both addresses still resolve to the same person,
and the address I actually use is the canonical one.

Signed-off-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>