IB: Fix typo in udev rule documentation

The proper syntax for udev rules is KERNEL==... instead of KERNEL=...

Signed-off-by: Bart Van Assche <bart.vanassche@gmail.com>
Reported-by: Lukasz Jurewicz <lukasz.jurewicz@gmail.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>