docs: i2c: piix4: Fix typos, add markup, drop link

[JD: Update the subject
     One more typo fixed
     Drop the link to lm-sensors' README, it's irrelevant]

Signed-off-by: Bruce Duncan <bwduncan@gmail.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@kernel.org>