module name in go.mod for v2

It should have v2 in `go.mod`
```
module github.com/pierrec/lz4/v2
```

and then this example works,
```Golang
import "github.com/pierrec/lz4/v2"

func test() {
    // ...
    text := lz4.NewReader(compressed)
    // ...
}
```