media: Documentation: st-vgxy61: Fix driver specific documentation not being included in the index

Fixes: 2378be89 ("media: Documentation: Add ST VGXY61 driver documentation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Benjamin Mugnier <benjamin.mugnier@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>