Highlighting in variables.md

Author: matthewjasper

src/variables.md

@@ -17,7 +17,7 @@ variable `y`).
 Methods that take either `self` or `Box<Self>` can optionally place them in a
 mutable variable by prefixing them with `mut` (similar to regular arguments):
 
-```
+```rust
 trait Changer: Sized {
     fn change(mut self) {}
     fn modify(mut self: Box<Self>) {}