Fix typos in comments for avifDecoderDataFindItem

Reverse AVIF_TRUE and AVIF_FALSE in the description of `alpha`.

src/read.c

diff --git a/src/read.c b/src/read.c
index 1e957f1..14f9041 100644
--- a/src/read.c
+++ b/src/read.c
@@ -3389,8 +3389,8 @@
     return AVIF_RESULT_OK;
 }
 
-// If alpha is AVIF_TRUE, searches for the primary color item (parentItemID is ignored in this case).
-// If alpha is AVIF_FALSE, searches for the auxiliary alpha item whose parent item ID is parentItemID.
+// If alpha is AVIF_FALSE, searches for the primary color item (parentItemID is ignored in this case).
+// If alpha is AVIF_TRUE, searches for the auxiliary alpha item whose parent item ID is parentItemID.
 // Returns the target item if found, or NULL.
 static avifDecoderItem * avifDecoderDataFindItem(avifDecoderData * data, avifBool alpha, uint32_t parentItemID)
 {