avutil/spherical: consolidate group doxy

Make it a bit easier to grasp the grouping when not
unnecessarily splitting comment blocks.

Additionally do not try to add lavu_video_spherical to itself, resolving
the following doxy warning:
  warning: Refusing to add group lavu_video_spherical to itself

libavutil/spherical.h

@@ -30,19 +30,14 @@
 #include <stdint.h>
 
 /**
- * @addtogroup lavu_video
- * @{
- *
  * @defgroup lavu_video_spherical Spherical video mapping
- * @{
- */
-
-/**
- * @addtogroup lavu_video_spherical
+ * @ingroup lavu_video
+ *
  * A spherical video file contains surfaces that need to be mapped onto a
  * sphere. Depending on how the frame was converted, a different distortion
  * transformation or surface recomposition function needs to be applied before
  * the video should be mapped and displayed.
+ * @{
  */
 
 /**
@@ -225,7 +220,6 @@ const char *av_spherical_projection_name(enum AVSphericalProjection projection);
  */
 int av_spherical_from_name(const char *name);
 /**
- * @}
  * @}
  */