Fix readme (#5513)

* Fix readme

* Fix spelling mistake
Co-Authored-By: Ndipiash <dmitriytemn@gmail.com>

packages/next/README.md

@@ -535,7 +535,7 @@ Router.beforePopState(({ url, as, options }) => {
 });
 ```
 
-If you return a falsy value from `beforePopState`, `Router` will not handle `popstate`;
+If you return a `false` value from `beforePopState`, `Router` will not handle `popstate`;
 you'll be responsible for handling it, in that case.
 See [Disabling File-System Routing](#disabling-file-system-routing).