Cleaned up the Table of Contents

Bill, step one, cleaned out the non-breaking spaces (nbsp) in favor of markdown lists.

I wasn't sure if you wanted the a few bullets within the list (prefaces & appendices) so I left them out for now. The [xAPI Spec](https://github.com/adlnet/xAPI-Spec/blob/master/xAPI.md) has Revision History as 1.0 and Contributors as a sub-item of section 2.0.