manifest: fix up help entry

src/manifest.c

@@ -126,6 +126,7 @@ static int manifest_main(void *ctx, struct apk_database *db, struct apk_string_a
 static struct apk_applet apk_manifest = {
 	.name = "manifest",
 	.help = "Show checksums of package contents",
+	.arguments = "PACKAGE...",
 	.open_flags = APK_OPENF_READ,
 	.main = manifest_main,
 };