close db in more correct way. less unsigned integers

src/main.c

@@ -16,6 +16,6 @@ int main(int argc, char *argv[]) {
   /* TODO: display server port */
   printf("Server is started!\n");
   server_main_loop(&server_data);
-  close_connection();
+  db_close_connection();
   return 0;
 }