Rename Bro to zeek

[besendorf]

In the Documentation there are references to "Bro" however the project was renamed to zeek. https://zeek.org/2018/10/11/renaming-the-bro-project/
Those references should be renamed. There are also attributes of events called bro. I'm not sure how to handle those, as data in MISP instances might still use them. Maybe it should be noted in the documentation that the attributes should not be used anymore and the zeek attribute should be used