Converts the size calculation in the explanation from a fenced code block to an indented one. I think it looks better when not rendered, and is the same rendered.
This type cannot have a well-defined size, because it needs to be arbitrarily
large (since we would be able to nest `ListNode`s to any depth). Specifically,
-```
-size of ListNode = 1 byte for head
- + 1 byte for the discriminant of the Option
- + size of ListNode
-```
+ size of `ListNode` = 1 byte for `head`
+ + 1 byte for the discriminant of the `Option`
+ + size of `ListNode`
One way to fix this is by wrapping `ListNode` in a `Box`, like so: