docs: i2c: writing-clients: properly name the stop condition

In I2C there is no such thing as a "stop bit". Use the proper naming: "stop
condition".

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