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>

Documentation/tutorial.txt

@@ -429,7 +429,9 @@ $ gitk --since="2 weeks ago" drivers/
 -------------------------------------
 
 allows you to browse any commits from the last 2 weeks of commits
-that modified files under the "drivers" directory.
+that modified files under the "drivers" directory.  (Note: you can
+adjust gitk's fonts by holding down the control key while pressing
+"-" or "+".)
 
 Finally, most commands that take filenames will optionally allow you
 to precede any filename by a commit, to specify a particular version