black circle code references in tutor misaligned

[jurgenvinju]

Describe the bug

• The references using a "black circle with a white number in them" work great. However, the black circle left of the explanation is centered. It should really be vertically aligned to the start of the explanation.

Note: This seems to be an issue with the Asciidoctor default stylesheet, not something under our control.

Via @Pieter007