fix suggested branch creation command when detaching head

fix suggested branch creation command when detaching head

Doing:

$ git checkout HEAD^

Generates the following message:

|warning: you are not on ANY branch anymore.
|If you meant to create a new branch from the commit, you need -b to
|associate a new branch with the wanted checkout.  Example:
|  git checkout -b <new_branch_name> HEAD^

Of course if the user does as told at this point the created branch
won't be located at the expected commit.  Reword this message a bit to
avoid such confusion.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>