docs: uio-howto.rst: use a code block to solve a warning

/devel/v4l/docs/Documentation/driver-api/uio-howto.rst:715: WARNING: Unexpected indentation.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/uio-howto.rst

@@ -711,7 +711,8 @@ The vmbus device regions are mapped into uio device resources:
 
 If a subchannel is created by a request to host, then the uio_hv_generic
 device driver will create a sysfs binary file for the per-channel ring buffer.
-For example:
+For example::
+
 	/sys/bus/vmbus/devices/3811fe4d-0fa0-4b62-981a-74fc1084c757/channels/21/ring
 
 Further information