README.md: API reference link is broken

`$ curl https://metacar-project.com/docs/modules/_index_.html`
```
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Error</title>
</head>
<body>
<pre>Cannot GET /docs/modules/_index_.html</pre>
</body>
</html>
```