Documentation/i2c: SMBus start signal is S not A

Just like all other I2C/SMBus commands, the start signal for the SMBus
Quick Command is S, not A.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Daniel Schaefer <git@danielschaefer.me>
Signed-off-by: Wolfram Sang <wsa@kernel.org>