Tutorial: use a bit longer sample filenames.

Tutorial: use a bit longer sample filenames.

Darrin Thompson noticed when he was showing off GIT to others
that the use of filenames "a" and "b" in the tutorial example
was unnecessarily confusing, especially with our "patch -p1"
prefix a/ and b/, without giving us any patch.  I was very
tempted to change them back to l/ and k/ prefixes, but decided
to restrain myself and update the tutorial instead ;-).

Signed-off-by: Junio C Hamano <junkio@cox.net>