Fix vertical alignment of code blocks in API reference page
After the changes in https://github.com/sketch-hq/SketchAPI/pull/605, code blocks in the API reference page no longer align correctly with their text counterparts.

Before:

<img width="1332" alt="Screenshot 2020-05-07 at 12 17 29" src="https://user-images.githubusercontent.com/3832/81283395-d8635180-905c-11ea-9d68-5831bc7ee420.png">

After:

<img width="1320" alt="Screenshot 2020-05-07 at 12 18 34" src="https://user-images.githubusercontent.com/3832/81283427-e87b3100-905c-11ea-8e70-83caffad1906.png">
