docs: i2c: i2c-topology: fix incorrect heading

"Etc" here was never meant to be a heading, it became one while converting
to ReST.

It would be easy to just convert it to plain text, but rather remove it and
add an introductory text before the list that conveys the same meaning but
with a better reading flow.

Fixes: ccf988b6 ("docs: i2c: convert to ReST and add to driver-api bookset")
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>