avutil/video_enc_params: fix doxy for av_video_enc_params_block()

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 4cba3e0f07)

libavutil/video_enc_params.h

@@ -136,8 +136,8 @@ typedef struct AVVideoBlockParams {
     int32_t delta_qp;
 } AVVideoBlockParams;
 
-/*
+/**
- * Get the block at the specified {@code idx}. Must be between 0 and nb_blocks.
+ * Get the block at the specified {@code idx}. Must be between 0 and nb_blocks - 1.
  */
 static av_always_inline AVVideoBlockParams*
 av_video_enc_params_block(AVVideoEncParams *par, unsigned int idx)