docs: Use Promise for dialog.showMessageBox (electron#21143)

Vilsafur · MarshallOfSound · commit a6a028594ddc · 2019-11-18T00:49:50.000-08:00
* Update updates.md

Use of the return promise for the use of `dialog.showMessageBox`.

* Update updates.md
diff --git a/docs/tutorial/updates.md b/docs/tutorial/updates.md
@@ -123,8 +123,8 @@ autoUpdater.on('update-downloaded', (event, releaseNotes, releaseName) => {
     detail: 'A new version has been downloaded. Restart the application to apply the updates.'
   }
 
-  dialog.showMessageBox(dialogOpts, (response) => {
-    if (response === 0) autoUpdater.quitAndInstall()
+  dialog.showMessageBox(dialogOpts).then((returnValue) => {
+    if (returnValue.response === 0) autoUpdater.quitAndInstall()
   })
 })
 ```
