documentation: add brief mention of cat-file to tutorial part I

documentation: add brief mention of cat-file to tutorial part I

I'd rather avoid git cat-file so early on, but the

	git-cat-file -p old-commit:/path/to/file

trick is too useful....

Also fix a nearby typo while we're at it.

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