Define generic for _app state (#8256)

packages/next/pages/_app.tsx

@@ -27,8 +27,9 @@ async function appGetInitialProps({
   return { pageProps }
 }
 
-export default class App<P = {}, CP = {}> extends React.Component<
-  P & AppProps<CP>
+export default class App<P = {}, CP = {}, S = {}> extends React.Component<
+  P & AppProps<CP>,
+  S
 > {
   static childContextTypes = {
     router: PropTypes.object,

packages/next/pages/_document.tsx

@@ -58,7 +58,7 @@ export default class Document<P = {}> extends Component<DocumentProps & P> {
   }
 
   /**
-   * `getInitialProps` hook returns the context object with the addition of `renderPage`. `
+   * `getInitialProps` hook returns the context object with the addition of `renderPage`.
    * `renderPage` callback executes `React` rendering logic synchronously to support server-rendering wrappers
    */
   static async getInitialProps({