Doc: cpufreq: Fix typo and outdated line

'sampling_rate_max' was removed with commit ef598549 ("[...] Remove
deprecated sysfs file sampling_rate_max"), so its line can be dropped
from governors.txt. And 'show_sampling_rate_min' is a typo: the sysfs
file is called 'sampling_rate_min'.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>