Fails to properly format go.mod file

Thanks very much for starting this.

If I create a simple `go.mod` file:

```
cat <<EOD > go.mod
module "rubbish.com/blah"

require (
	"github.com/kisielk/gotool" v1.0.0
)
```

open in Vim and then `gg=G` to reformat I'm left with:

```
module "rubbish.com/blah"

require (
    "github.com/kisielk/gotool" v1.0.0
    )
```

Am I doing something wrong here?

Thanks