Doc/checkout: checking out using @{-N} can lead to detached state

Doc/checkout: checking out using @{-N} can lead to detached state

@{-N} is a syntax for the N-th last "checkout" and not the N-th
last "branch". Therefore, in some cases using `git checkout @{-$N}`
DOES lead to a "detached HEAD" state. This can also be ensured by
the commit message of 75d6e552a (Documentation: @{-N} can refer to
a commit, 2014-01-19) which clearly specifies how @{-N} can be used
to refer not only to a branch but also to a commit.

Correct the misleading sentence which states that @{-N} doesn't
detach HEAD.

Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>