cfg80211: fix example REG_RULE usage in Documentation

It's just an example, but lets make it look more real to don't confuse
people about possible REG_RULE usage. Channels are 20 MHz wide, so start
and end frequencies are 10 MHz away from the center one.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>