Make pattern matching exercise code editable (#2786)

I swear I already did this one...

@djmitche it looks like you had removed this `editable` after I last
added it, was there some reason for that?

Author: randomPoison

src/pattern-matching/exercise.md

@@ -46,7 +46,7 @@ evaluate to `85`. We represent this as a much bigger tree:
 
 In code, we will represent the tree with two types:
 
-```rust
+```rust,editable
 {{#include exercise.rs:Operation}}
 
 {{#include exercise.rs:Expression}}