HACKING: correct output folder in the format-patch example

Signed-off-by: Aidan Hobson Sayers <aidanhs@cantab.net>
Reviewed-by: Zoran Zaric <zz@zoranzaric.de>
[rlb@defaultvalue.org: adjust commit message.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>