fixed printing argv[0]

src/main.c

@@ -26,7 +26,7 @@ int main(int argc, char *argv[]) {
 				break;
 			case '?':
 				printf("unknown option: %c\n", optopt);
-				printf("usage: %c -l -n DATE EVENT_NAME\n", argv[0]);
+				printf("usage: %s -l -n DATE EVENT_NAME\n", argv[0]);
 				break;
 		}
 	}