user-manual: remove temporary branch entry from todo list

In the section on "How to check out a different version of a project" the "new" branch is used as a temporary branch. A detached HEAD was not used since it was a new feature introduced just a couple weeks prior. The section could be changed to use and explain a detached HEAD, except that would increase the learning curve early in the manual. Detached HEADs are discussed a couple sections later under "Examining an old version without creating a new branch". Let's declare that it is a bad idea to rewrite the example that uses a temporary branch to do the sightseeing on a detached HEAD. Signed-off-by: Stephen P. Smith <ischis2@cox.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>