push doc: move mention of "tag <tag>" later in the prose

push doc: move mention of "tag <tag>" later in the prose

This change will be followed-up with a subsequent change where I'll
change both sides of this mention of "tag <tag>" to be something
that's best read without interruption.

To make that change smaller, let's move this mention of "tag <tag>" to
the end of the "<refspec>..." section, it's now somewhere in the
middle.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>