Remove router property from _app docs

It’s an inconsistent result, users should use ctx instead. At a later time we’ll normalize the properties passed into _app.js its getInitialprops to be consistent with pages.

packages/next/README.md

@@ -1052,7 +1052,7 @@ import React from 'react'
 import App, { Container } from 'next/app'
 
 export default class MyApp extends App {
-  static async getInitialProps({ Component, router, ctx }) {
+  static async getInitialProps({ Component, ctx }) {
     let pageProps = {}
 
     if (Component.getInitialProps) {