fix: refine demo.c

examples/c/demo.c

@@ -63,7 +63,7 @@ int main(int argc, char *argv[]) {
 
   TAOS *taos = taos_connect(argv[1], "root", "taosdata", NULL, 0);
   if (taos == NULL) {
-    printf("failed to connect to server, reason:%s\n", "null taos"/*taos_errstr(taos)*/);
+    printf("failed to connect to server, reason:%s\n", taos_errstr(NULL));
     exit(1);
   }
   for (int i = 0; i < 100; i++) {