git-apply --whitespace=fix: fix off by one thinko

When a patch adds a whitespace followed by end-of-line, the
trailing whitespace error was detected correctly but was not
fixed, due to misconversion in 42ab241 (builtin-apply.c: do not
feed copy_wsfix() leading '+').

Signed-off-by: Junio C Hamano <gitster@pobox.com>