Update no-document-title.md (#6832)

`router` is not needed in the scope

errors/no-document-title.md

@@ -15,7 +15,7 @@ import Head from 'next/head'
 import React from 'react'
 
 export default class MyApp extends App {
-  static async getInitialProps ({ Component, router, ctx }) {
+  static async getInitialProps ({ Component, ctx }) {
     let pageProps = {}
 
     if (Component.getInitialProps) {