This PR fixes the buggy `Head.propTypes` here:
https://github.com/zeit/next.js/blob/v8.0.0-canary.3/packages/next-server/lib/head.js#L107
Currently, `Head.propTypes` allows one child node like this:
```jsx
import Head from 'next/head'
// …
<Head>
<title>Title</title>
</Head>
```
But more than one child node mistakenly causes a prop type error like this:
```jsx
<Head>
<title>Title</title>
<meta name="description" content="Description." />
</Head>
```
```
Warning: Failed prop type: Invalid prop `children` supplied to `Head`.
```
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