Fix links to API docs in demos

[acse-ej321]

As @ddundo recently pointed out, you need ~. in front of :class: references in order to properly link to the API docs. These are absent in quite a few of the documentation references in the demos.

[ddundo]

We already have an open issue for this mesh-adaptation/docs#5. I've been thinking of just sprinting through demos and fixing this but I'd like to get a reliable doc builder first - hopefully after mesh-adaptation/docs#52 merges :)