Fix GitHub casing in slides and a little consistent use of YAML example, links

A few slipped through after you [fixed the filename issue for those looking on Windows](https://github.com/github/teach.github.com/issues/139).

- Pump up the H in GitHub
- And the YAML all caps as is polite.

The YAML slide says you can use three or more hyphens, and it was odd the first use was four, and nowhere else. 

Signup link appears to be the only one loaded with http for its display. Other links just jump right into the subdomain part.