tag: fix typo in nested tagging hint

In eea9c1e78f (tag: advise on nested tags, 2019-04-04), tag was taught
to hint at the user if a nested tag is made. However, this message had a
typo and it said "The object referred to by your new is...", which was
missing a "tag" after "new". Fix this message by adding the "tag".

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
