Highlight a variable and a code block

This PR highlights a variable and a code block in closures.md.

src/fn/closures.md

@@ -1,9 +1,9 @@
 # Closures
 
 Closures are functions that can capture the enclosing environment. For
-example, a closure that captures the x variable:
+example, a closure that captures the `x` variable:
 
-```Rust
+```rust
 |val| val + x
 ```