Fix broken links

**Before this pull request**

```
$ bundle exec rake test
Running [Links, Images] checks on ./output on *.html...

Checking 415 external links...
Ran on 174 files!

./output/guides/getting-started/index.html: External link http://nanoc.stoneship.org/ failed: 409 No error

./output/v3/enterprise/management_console/index.html: External link https://help.github.com/enterprise/11.10.340/admin/articles/setting-the-management-console-password/ failed: 404 No error

rake aborted!
HTML-Proofer found 2 failures!
/Users/j/github/developer.github.com/Rakefile:14:in `block in <top (required)>'
Tasks: TOP => test
(See full trace by running task with --trace)
```

**After this pull request**

```
$ bundle exec rake test
Running [Images, Links] checks on ./output on *.html...

Checking 415 external links...
Ran on 174 files!

HTML-Proofer finished successfully.
```

/cc @gjtorikian