i2c: gpio: fault-injector: better SPHINX style in docs

We don't use SPHINX currently in I2C documentation; but preparing the
I2C fault injector docs already is a good idea since it makes it easier
to read. Especially as new stuff is going to be added soon.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>