notes: fix minimum number of parameters to "copy" subcommand

notes: fix minimum number of parameters to "copy" subcommand

The builtin/notes.c::copy() function is prepared to handle either
one or two arguments given from the command line; when one argument
is given, to-obj defaults to HEAD.

bbb1b8a3 ("notes: check number of parameters to "git notes copy"",
2010-06-28) tried to make sure "git notes copy" (with *no* other
argument) does not dereference NULL by checking the number of
parameters, but it incorrectly insisted that we need two arguments,
instead of either one or two.  This disabled the defaulting to-obj
to HEAD.

Correct it.

Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>