Fix typos and syntax errors in this.md

This fix was already merged in (in https://github.com/raganwald/homoiconic/pull/36 ), but was later accidentally clobbered by commit b2b5c4ad6069.

This pull request fixes 4 issues with this.md:

* An English typo in the second paragraph.
* A call to _.extend was missing the _. prefix.
* A syntax error in the aFourthObject example (missing comma).
* A logic error in the aFourthObject example. I had trouble determining the intent of this example, but b() === aFourthObject; did not evaluate to true as the comment indicated it should, so I changed the code so it would evaluate to true. Maybe you had a different bit of logic in mind, though, so you should fix it the way you originally intended.