Fix copy-and-paste error in comment for RGB to YUV

The comment should say AVIF_CHROMA_DOWNSAMPLING_xxx, not
AVIF_CHROMA_UPSAMPLING_xxx.
diff --git a/include/avif/avif.h b/include/avif/avif.h
index 5800b33..9b34159 100644
--- a/include/avif/avif.h
+++ b/include/avif/avif.h
@@ -547,7 +547,7 @@
 //
 // * YUV depth: 8
 // * RGB depth: 8
-// * rgb.chromaDownsampling: AVIF_CHROMA_UPSAMPLING_AUTOMATIC, AVIF_CHROMA_UPSAMPLING_FASTEST
+// * rgb.chromaDownsampling: AVIF_CHROMA_DOWNSAMPLING_AUTOMATIC, AVIF_CHROMA_DOWNSAMPLING_FASTEST
 // * One of the following combinations (avifRGBFormat to avifPixelFormat/MC/Range):
 //   *  BGRA            to  YUV400        /  x  /[Full|Limited]
 //   *  BGRA            to [YUV420|YUV422]/[5|6]/[Full|Limited]