Fix the deprecation warning

https://github.com/atom/electron/pull/3424

main.js

@@ -26,7 +26,7 @@ app.on('ready', function() {
   mainWindow = new BrowserWindow({width: 800, height: 600});
 
   // and load the index.html of the app.
-  mainWindow.loadUrl(`file://${__dirname}/index.html`);
+  mainWindow.loadURL(`file://${__dirname}/index.html`);
 
   // Open the DevTools.
   mainWindow.webContents.openDevTools();