Incorrect Tailwind config

I won't fork for such a small change.

Your `tailwind.config.js` is pointing to the `pages` folder, but you are implementing next's experimental `app` folder so no tailwind styling is being generated.
```js
(...)
content: [
"./pages/**/*.{js,ts,jsx,tsx}",
(...)
```

Should be
```js
(...)
content: [
"./app/**/*.{js,ts,jsx,tsx}",
(...)
```

Open tailwind.config.js and inspect the content paths used for the Next app. Update the configuration to cover the app folder as described in the issue, then verify that Tailwind styling is generated for files under app.

索引モデルが issue の本文から書いたものです。