Documentation: "pack-file" is not literal in unpack-objects

Make it clear that "pack-file" is not to be spelled as is in the
unpack-objects usage.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>