FDE disk setup instructions are misleading when installing from USB

[tetrahedra]

When installing from a USB thumb drive, the machine's internal HDD 
usually shows up as sd0 and the thumb drive as sd1.

However, the FDE installation instructions 
<https://www.openbsd.org/faq/faq14.html#softraidFDE> suggest that we 
should overwite the first 1MB of sd1 with zeros:

# dd if=/dev/zero of=/dev/rsd1c bs=1m count=1

If the user is installing from a USB drive, this will lead to them 
overwriting the USB drive, rather than the pseudo-device.