This is the body of a boilerplate template that I've used the last two times I've changed jobs:

Please accept this letter as formal notification that I am leaving my position with [company name] on [full date of departure]. After much consideration I have decided to accept another job offer. My new position will enhance my career growth and will expose me to challenges and opportunities that I believe are in my best interest.

I value the professional relationships I have made with my coworkers, as well as yourself. Because of this, if there is anything I can do in my last days of employment to help transition my work, please let me know.