Update Next.js custom app documentation link (#772)

README.md

@@ -173,7 +173,7 @@ Some tools may report this an error. See [Issue #14](https://github.com/garmeeh/
 
 `NextSeo` enables you to set some default SEO properties that will appear on all pages without needing to do include anything on them. You can also override these on a page by page basis if needed.
 
-To achieve this, you will need to create a custom `<App>`. In your pages directory create a new file, `_app.js`. See the Next.js docs [here](https://github.com/zeit/next.js/#custom-app) for more info on a custom `<App>`.
+To achieve this, you will need to create a custom `<App>`. In your pages directory create a new file, `_app.js`. See the Next.js docs [here](https://nextjs.org/docs/advanced-features/custom-app) for more info on a custom `<App>`.
 
 Within this file you will need to import `DefaultSeo` from `next-seo` and pass it props.