libalpm: clarify alpm_download_event_completed_t status

The comment makes it seem that the result itself is an error code. But
all it does is simply return -1 to indicate an error occured;

Signed-off-by: Allan McRae <allan@archlinux.org>

lib/libalpm/alpm.h

@@ -730,7 +730,7 @@ typedef struct {
 	/* download result code:
 	 *    0 - download completed successfully
 	 *    1 - the file is up-to-date
-	 *   negative - error code
+	 *   -1 - error
 	 */
 	int result;
 } alpm_download_event_completed_t;