Fix dox2go import path in PDF renderer.

I was getting the following error when using the library prior to this change:

```
../../../adkennan/dox2go/pdf/pdf_page.go:11:2: cannot find package "dox2go" in any of:
        /usr/lib/go/src/pkg/dox2go (from $GOROOT)
        /home/zach/go/src/dox2go (from $GOPATH)
```