close %block tag, update disqus stuff

Update of 'Creating a theme' so to work with v5.4+

- __disqus__ stuff wasn't working, updated with v5.4-style disqus tags
- A __%block__ tag was unclosed (line 239)
- Very minor typo fixed (line 183)
Merge pull request #371 from quijot/patch-4

close %block tag, update disqus stuff