Updated note on how to run partest

It appears that something in the `partest` config has changed since the guide was written. If you invoke `partest` from inside of the `test` folder you will get the following complaints:

```bash
Buildfile: build.xml does not exist!
Build failed
cat: /Users/hartmann/dev/scala/test/build/pack/partest.properties: No such file or directory
Error: Could not find or load main class scala.tools.partest.nest.ConsoleRunner
```

If you run in from the root then everything works as you'd expect.