docs: fix app.on('session-created' example (#19713)

It does not have the 'event' argument.

Refs: https://github.com/electron/electron/pull/12123
Refs: https://github.com/electron/electron/pull/15236
Refs: https://github.com/electron/electron/issues/15203

docs/api/app.md

@@ -390,7 +390,7 @@ Emitted when Electron has created a new `session`.
 ```javascript
 const { app } = require('electron')
 
-app.on('session-created', (event, session) => {
+app.on('session-created', (session) => {
   console.log(session)
 })
 ```