Correct "umount" to "unmount" in a couple of places

This is admittedly a nitpicky change, but umount is the command that
performs an *unmount*. So if we are talking about unmounting something
we should phrase it that way.

Signed-off-by: Simon Howard <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>