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>

Documentation/driver-api/media/camera-sensor.rst

@@ -144,9 +144,7 @@ of the device. This is because the power state of the device is only changed
 after the power state transition has taken place. The ``s_ctrl`` callback can be
 used to obtain device's power state after the power state transition:
 
-.. c:function::
-
-	int pm_runtime_get_if_in_use(struct device *dev);
+.. c:function:: int pm_runtime_get_if_in_use(struct device *dev);
 
 The function returns a non-zero value if it succeeded getting the power count or
 runtime PM was disabled, in either of which cases the driver may proceed to