bup-init.md: update after dropping automatic ~/.bup init.

Signed-off-by: Zoran Zaric <zz@zoranzaric.de>
[rlb@defaultvalue.org: adjust commit message; remove escape from
 BUP\_DIR after list discussion; remove additional "bup init" from
 README.md example.]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>