Move struct update syntax to speaker notes

[randomPoison]

I don't think we need to cover struct update syntax in the slide on structs. The Default trait slide covers this later on, and in a context that makes more sense, i.e. using it in combination with Default, which is the main usage of the syntax. This would give us a little more breathing room in day 1. I've instead moved it to the speaker notes, so we can still talk about it if students ask.