Fixed some Githubs's markdown rendering problems in part 2 of the documentation

[diegoreico]

There were some titles in the doc that had wrong spacing for Github's markdown rendering engine.

In the web browser everybody was seeing something like

### 2.5 Retrieval of Statements

Instead of

2.5 Retrieval of Statements

[garemoko]

Thanks so much for your contribution. Github changed their markdown rending last week and it has broken a lot of documents, not just the xAPI spec.

The issues you're seeing can also be resolved by using correctly closed anchor tags, which I think is the better fix. There's already a PR that fixes all of the documents except contributing.md here: #1034

And another PR addressing contributing.md here: #1036

So, thanks again for your contribution, but in this case a fix is already in the works.

[andyjohnson]

Hey Diego, I'm guessing these are covered in other merges (and there is just a discrepancy in number/style of return carriages). Let's wait on @aakoch as he had a couple merges and if yours haven't been addressed, could you work off the latest merge? Thanks!

[garemoko]

Looks like the branch for this has now been deleted as it's now saying "unknown repository". We should probably just close this PR.