Fix comment nits (#507)

diff --git a/examples/avif_example_encode.c b/examples/avif_example_encode.c
index 7bcc01c..a355799 100644
--- a/examples/avif_example_encode.c
+++ b/examples/avif_example_encode.c
@@ -35,7 +35,7 @@
     // * avifImageSetMetadataXMP()
     // * yuvRange
     // * alphaRange
-    // * alphaPremultiply
+    // * alphaPremultiplied
     // * transforms (transformFlags, pasp, clap, irot, imir)
 
     if (encodeYUVDirectly) {