I don't have an example, but I would suggest writing either nothing significant in it at all, "I've enjoyed my time at the B52s..." and so on...

or

give your reasons for leaving, provided they are a pull factor, rather than a push one. In other words "I am seeking further development of my project management career and seeking new life and new civilization" without adding : "and I can't do that here". That will portray you in your exit as a positive ambitious go-getter person, on the up.

I suppose the main recommendation is simply not to write anything that you may subsequently regret: either against the company or any individuals within it.

Hope this helps
dave

PS: Do not tell them where you are going until you have your feet under your new desk.