Update generic.conf - add directional antenna example

[Firehammer047]

The manual seems to be missing this explanation, so I added an example for setting the direction of the antenna in the PBEACON

[wb2osz]

Thank you for your suggestion.
Initially, I tried adding too much explanation and too many examples to the sample configuration file.
Experience showed that it was too complicated and cluttered and caused confusion for many people.
It made troubleshooting more difficult.
My new approach is to keep it very short and simple for the most common cases.
Users are directed to the documentation to learn about more advanced features.