Expanded ToC example

[milosimpson]

Expanded the ToC doc to make it more clear that the {:toc} has to be used in conjunction with a markdown list, i.e. {:toc} is not a "valid" expression all by itself.

Ala https://stackoverflow.com/questions/48071478/table-of-contents-in-markdown-using-only-the-standard-out-of-the-box-github-p

[gettalong]

Thanks for your contribution - I have applied it.