docs: i2c: smbus-protocol: remove unneeded colons from table

These colons are not needed: the columns already nicely separate the
symbols from their description. They are also inconsistently preceded by
whitespace.

Remove the colons completely to simplify and clean up.

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