Fix bootloader not being written in Plan9

note, May 11, 2023 on Mitja Felicijan's blog

If the bootloader is not being written to a disk when installing 9front on real @@ -8,21 +8,18 @@ harware try clearing first sector of the disk with the following command.

# If command above doesn't work try this one, wait couple of seconds and
 # press delete key to stop the command.
 cat </dev/zero >/dev/sd*/data
-