DOC: fix sphinx build dir in CONTRIBUTING

[hagenw]

We need to hard code the sphinx build dir inside conf.py at the moment as we need the path to store the example WAV files.
This means the path when building the docs we mention in CONTRIBUTING.rst has to match the path we use inside conf.py which was not the case before.

In addition, I also fixed to instances that were still mentioning audfoo instead of auglib in CONTRIBUTING.rst.