Add syntax highlighting (#3123)

Sources/ComposableArchitecture/Effects/Cancellation.swift

@@ -124,7 +124,7 @@ extension Effect {
 /// If the operation is in-flight when `Task.cancel(id:)` is called with the same identifier, the
 /// operation will be cancelled.
 ///
-/// ```
+/// ```swift
 /// enum CancelID { case timer }
 ///
 /// await withTaskCancellation(id: CancelID.timer) {