Correct claim about number of packs per backup

Correct claim about number of packs per backup

The current sentence implies that there is only one pack file per backup
run, which is not necessarily correct.

Because of the "constants" max_pack_size and max_pack_objects that are
used as limits to pack sizes, we may end up with more than one pack per
backup if there is a lot of data that needs to be stored.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
[rlb@defaultvalue.org: adjust commit summary]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>