Fix help message of `avifdec`

`--index` takes a value, but this is missing. So fix this.

apps/avifdec.c

diff --git a/apps/avifdec.c b/apps/avifdec.c
index 9e74504..ba67d2e 100644
--- a/apps/avifdec.c
+++ b/apps/avifdec.c
@@ -37,7 +37,7 @@
     printf("    -u,--upsampling U : Chroma upsampling (for 420/422). automatic (default), fastest, best, nearest, or bilinear\n");
     printf("    -r,--raw-color    : Output raw RGB values instead of multiplying by alpha when saving to opaque formats\n");
     printf("                        (JPEG only; not applicable to y4m)\n");
-    printf("    --index           : When decoding an image sequence or progressive image, specify which frame index to decode (Default: 0)\n");
+    printf("    --index I         : When decoding an image sequence or progressive image, specify which frame index to decode (Default: 0)\n");
     printf("    --progressive     : Enable progressive AVIF processing. If a progressive image is encountered and --progressive is passed,\n");
     printf("                        avifdec will use --index to choose which layer to decode (in progressive order).\n");
     printf("    --no-strict       : Disable strict decoding, which disables strict validation checks and errors\n");