banned.h: fix vsprintf()'s ban message

banned.h: fix vsprintf()'s ban message

In cc8fdaee1e (banned.h: mark sprintf() as banned, 2018-07-24), both
'sprintf()' and 'vsprintf()' were marked as banned functions. The
non-variadic macro to ban 'vsprintf' has a typo which says that
'sprintf', not 'vsprintf' is banned. The variadic version does not have
the same typo.

Fix this by updating the explicit form of 'vsprintf' as the banned
version of itself, not 'sprintf'.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>