mx25l.4: Document the correct disk device path

This was true at time of commit, but the path was changed 2 weeks later
to just be the /dev/flash/spiN name, without updating the manpage.

Reported by: David Gilbert <dgilbert@daveg.ca>
Fixes: 68dd77957786 ("Give the mx25l device sole ownership of the name /dev/flash/spi* ...")
MFC after: 1 week