Update xAPI.md
Updated Section 4.1.4 Objects
Updated vocabulary mistakes
actDef optional/required + url

* updated activity definition optional/required column (added "recommended")
* re-named url -> moreInfo
* trimmed interaction info row
Merge pull request #6 from bscSCORM/objects

actDef optional/required + url
Merge pull request #243 from andyjohnson/master

Edits to Section 4.1.4 Objects