media: Documentation: staging/ipu3-imgu: Fix reference file name

The intel-ipu3.h intended-to-be-uAPI header is currently under
drivers/staging/media/ipu3/include/, not include/uapi/linux.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/v4l-drivers/ipu3.rst

@@ -358,7 +358,7 @@ The source can be located under hal/intel directory.
 References
 ==========
 
-.. [#f5] include/uapi/linux/intel-ipu3.h
+.. [#f5] drivers/staging/media/ipu3/include/intel-ipu3.h
 
 .. [#f1] https://github.com/intel/nvt