docs: correcting main example in WebRequest documentation (#36113)

Correcting main example

The entry `*://electron.github.io` is invalid and produces an exception. i.e.

> UnhandledPromiseRejectionWarning: TypeError: Invalid url pattern *://electron.github.io: Empty path.

Setting a valid path to resolve this issue

docs/api/web-request.md

@@ -28,7 +28,7 @@ const { session } = require('electron')
 
 // Modify the user agent for all requests to the following urls.
 const filter = {
-  urls: ['https://*.github.com/*', '*://electron.github.io']
+  urls: ['https://*.github.com/*', '*://electron.github.io/*']
 }
 
 session.defaultSession.webRequest.onBeforeSendHeaders(filter, (details, callback) => {