Tpo in Composition section

[BenGitsCode]

Sometimes, we want build more complex object by using specific instances of other
- objects. We can use composition to acheive this. In this case, we will add
+ objects. We can use composition to achieve this. In this case, we will add
 instances of the `Song` class to the tracks of our album.

[jrhorn424]

For those with tired eyes, it's "achieve".