Fix a bad code block.

Rob Rix · Rob Rix · commit 9e0e87fa957e · 2014-11-23T18:44:03.000-05:00
diff --git a/README.md b/README.md
@@ -21,8 +21,7 @@ let value = box.value
 
 Changing the value of a `MutableBox`:
 
-```
-swift
+```swift
 // Mutation:
 let mutableBox = MutableBox(1)
 mutableBox.value = 2
