Fix typos and syntax errors in this.md

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.