documentation: mention gitk font adjustment in tutorial

Kind of silly, but the font I get by default in gitk makes it mostly
unusable for me, so this is the first thing I'd want to know about.
(But maybe there's a better suggestion than just Ctrl-='ing until
satisfied.)

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>