Make code in expression evaluation exercise editable (google#2601)

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}}
@@ -68,7 +68,7 @@ get the tests to pass one-by-one. You can also skip a test temporarily with
 fn test_value() { .. }
 ```
 
-```rust
+```rust,editable
 {{#include exercise.rs:Operation}}
 
 {{#include exercise.rs:Expression}}