rfkill: Correctly document rkill subfolder range as >= 0 in sysfs-class-rfkill

The first subfolder is rfkill0, hence rfkillX (X >= 0).
Also fix two trivial typos.

Signed-off-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>