Code highlight a variable

This PR code highlights a variable in closures.md.

src/fn/closures.md

@@ -1,7 +1,7 @@
 # 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
 |val| val + x