Listing 2.20 and 2.21's code may need to be switched

The title for `Listing 2.19` and `Listing 2.20` are identical though the one for `Listing 2.20` appears to be the one that's incorrect.
It appears the code for `Listing 2.21` needs to instead be swapped with the code in `Listing 2.20` based on both the text surrounding both examples and the `Output 2.11` content which appears to align with what is currently the code of `Listing 2.20`.

Despite the above aligning with the text, the test names also don't line up with the content so I'm not sure if the text even should be in that order to begin with given the previous commits to those sections were movement from 2.09.

If someone can confirm the final intended order / changes desired I'm happy to submit a PR to correct this!
Really appreciate the book and it's availability online @MarkMichaelis !