docs: i2c: smbus-protocol: use proper names for ACK and NACK

Use the proper ACK and NACK naming from the I2C specification instead of
"accept" and "reverse accept".

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>