user-manual: reset to ORIG_HEAD not HEAD to undo merge

As Linus pointed out recently on the mailing list,

git reset --hard HEAD^

doesn't undo a merge in the case where the merge did a fast-forward.  So
the rcommendation here is a little dangerous.

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