Adjust "extensions" capitalization in headers

[brianjmiller]

Resolves #506

[brianjmiller]

I threw this PR together to satisfy #506 based on it stating that there had already been consensus in the group meeting from that date a long time ago, but I now question whether this change makes sense in light of the fact that most of the other xAPI related concepts don't share this same capitalization. For instance actor vs. Actor, or completion vs. Completion. I would still be in favor of making sure the document is consistent, but I think that would mean capitalizing the header in section 9.6.3 instead.

[MrBillMcDonald]

The Group determined that it would be more consistent to rename header with capitalization (to be consistent with other existing headers). The group will revisit capitalization of all headers with xAPI terms in a future release.