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>