Show an example in the Sway Book where a `match` arm is a `const`

This is a new feature that would nice showcase in the book. Example:
```rust
const A:u64 = 0;
const B:u64 = 1;
...
match value {
A => { .. },
B => { .. },
_ => { .. },
}

Start in the Sway Book and find the section covering match expressions or constants. Add a book example showing const names used as match arms, based on the issue's snippet, and verify that the documentation builds and the example renders correctly.

Written by the indexing model from the issue text.