bup-prune-older.md: remove fictional --keep option

bup-prune-older.md: remove fictional --keep option

The documentation lists the "--keep" option, but the actual command
doesn't implement it:

 $ bup prune-older --unsafe --keep 7d,1y,10y,100y
 usage: bup prune-older [options...] [BRANCH...]
 [...]
 error: option --keep not a unique prefix

Signed-off-by: Alexander Barton <alex@barton.de>
[rlb@defaultvalue.org: shorten summary line]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>