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

Documentation/git-checkout.txt

@@ -145,8 +145,8 @@ $ git checkout hello.c            <3>
 ------------
 +
 <1> switch branch
-<2> take out a file out of other commit
-<3> restore hello.c from HEAD of current branch
+<2> take a file out of another commit
+<3> restore hello.c from the index
 +
 If you have an unfortunate branch that is named `hello.c`, this
 step would be confused as an instruction to switch to that branch.