User-Agent Example

I was venturing into the wonderful world of the GitHub API tonight and was sent to this part of the docs through an error message I got about a User Agent. I found the docs a little confusing. From what I learned, the docs could be more clear by specifying that the user agent must be a _header_ (it currently says string) and written like `user-agent` since it is a header and has to be hyphenated (it currently says User Agent). I also thought an example would be helpful as well. This PR makes little changes to this effect.

![screen shot 2013-06-11 at 11 05 51 pm](https://f.cloud.github.com/assets/1305617/641157/c396b800-d326-11e2-9205-c699c26db8b2.png)

Also, it says "All API requests MUST include" but then says "If you are hitting the API without authentication." If it MUST include something then how can you possibly be hitting the API without it? Maybe I'm just too much a n00b in the world of APIs but I thought it was worth mentioning. 

Woo! :cat2: