manpages: italicize gitk's name (where it was in teletype font)

manpages: italicize gitk's name (where it was in teletype font)

The name `gitk` is sometimes meant to be entered at the command
prompt, but most uses are just referring to the program with that
name (not the incantation to start it).

Signed-off-by: Jonathan Nieder <jrnieder@uchicago.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>