Update install instructions in readme

This doesn't work:

```shell
go get github.com/go-ozzo/ozzo-validation
go: github.com/go-ozzo/ozzo-validation: no matching versions for query "upgrade"
```

This _does_ work:

```shell
go get github.com/go-ozzo/ozzo-validation/v4
go: downloading github.com/go-ozzo/ozzo-validation/v4 v4.3.0
go: added github.com/go-ozzo/ozzo-validation/v4 v4.3.0
```