docs: correct receive.advertisePushOptions default

docs: correct receive.advertisePushOptions default

In commit c714e45 ("receive-pack: implement advertising and receiving
push options", 2016-07-14), receive-pack was taught to (among other
things) advertise that it understood push options, depending on
configuration. It was documented that it advertised such ability by
default; however, it actually does not. (In that commit, notice that
advertise_push_options defaults to 0, unlike advertise_atomic_push which
defaults to 1.)

Update the documentation to state that it does not advertise the ability
by default.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>