commit: quote the user name in the example

commit: quote the user name in the example

If the user runs

 git config --global user.name Your Name

as suggested, user.name will be set to "Your".  With this patch, the
suggested command will be

 git config --global user.name "Your Name"

which will set user.name to "Your Name" and hopefully help users avoid
the former mistake.

Signed-off-by: Matt Kraai <kraai@ftbfs.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>