media: camera-sensor.rst: fix c:function build warning

The :c:function: tag evaluation depends on Sphinx version 2 or 3.
Use a syntax that should work with both versions.

Link: https://lore.kernel.org/linux-media/e4214991b7d59c0d7aa4e6e48833dc1809c47d47.1616484262.git.mchehab+huawei@kernel.org


Fixes: c0e3bcb2 ("media: camera-sensor.rst: fix a doc build warning")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>