fix: use optional plural form (oras-project#659)

Signed-off-by: Billy Zha <jinzha1@microsoft.com>

Author: qweeah

cmd/oras/discover.go

@@ -105,7 +105,11 @@ func runDiscover(opts discoverOptions) error {
 		return printDiscoveredReferrersJSON(desc, refs)
 	}
 
-	fmt.Println("Discovered", len(refs), "artifacts referencing", repo.Reference)
+	if n := len(refs); n > 1 {
+		fmt.Println("Discovered", n, "artifacts referencing", repo.Reference)
+	} else {
+		fmt.Println("Discovered", n, "artifact referencing", repo.Reference)
+	}
 	fmt.Println("Digest:", desc.Digest)
 	if len(refs) > 0 {
 		fmt.Println()