Unnecessary import in 8.2.2

The `string` import is unnecessary (in the `Defining a Parser` section) as `Parser.string` is used in the rest of the code. 

```elm
import Url.Parser as Parser exposing ((</>), Parser, s, string)
```

should be

```elm
import Url.Parser as Parser exposing ((</>), Parser, s)
```

This import is also present in `Listing 8.4`.