git-checkout.txt: fix incorrect statement about HEAD and index
git-checkout.txt: fix incorrect statement about HEAD and index

The command "git checkout" checks out from the index by default, not
HEAD (the introducing comment were correct, but the detailled
explanation added below were not).

Signed-off-by: Matthieu Moy <d4d2cc2a6ec7b2aa587d4887cc5ed200f9b685a1@imag.fr>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git