media: ipu3.rst: fix a build warning

Sphinx 2.4.4 produces this warning:

	Documentation/admin-guide/media/ipu3.rst:235: WARNING: Title underline too short.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/ipu3.rst

@@ -232,7 +232,8 @@ The IPU3 ImgU pipelines can be configured using the Media Controller, defined at
 :ref:`media_controller`.
 
 Running mode and firmware binary selection
--------------------------
+------------------------------------------
+
 ImgU works based on firmware, currently the ImgU firmware support run 2 pipes in
 time-sharing with single input frame data. Each pipe can run at certain mode -
 "VIDEO" or "STILL", "VIDEO" mode is commonly used for video frames capture, and