`Drop` slide should mention drop glue by name in speaker notes

The separation of `Drop` (which can add custom logic before fields are dropped) from the built-in logic that drops fields afterward is a critical part of understanding how object destruction works in Rust. The latter half is known as "drop glue", as [mentioned by the std docs](https://doc.rust-lang.org/std/ops/trait.Drop.html).

I always mention this name while teaching so that students have a name for this important phase of object destruction; we should mention it in the speaker notes so that students can come back to it and don't need to read the std docs to find out what this is called.