Copyright notice in code snippets

Most code snippets in the course show a copyright header of the form

```rust
// Copyright 20xx Google LLC
// SPDX-License-Identifier: Apache-2.0
```

This is a problem since it's visual noise that takes up valuable screen space.

Some code blocks don't show this header though. My preliminary investigation suggests that blocks opened by only

```rust

don't show the copyright header whereas those opened by more complex fences, such as

```rust,editable

do show it.