Fixes task cancellation sample code

The code calls `isCancelled` as if it were a property, but it's a Type property on `Task`.  So I changed `isCancelled` to `Task.isCancelled`

Author: Diggory

TSPL.docc/LanguageGuide/Concurrency.md

@@ -713,7 +713,7 @@ let photos = await withTaskGroup(of: Optional<Data>.self) { group in
     let photoNames = await listPhotos(inGallery: "Summer Vacation")
     for name in photoNames {
         group.addTaskUnlessCancelled {
-            guard isCancelled == false else { return nil }
+            guard Task.isCancelled == false else { return nil }
             return await downloadPhoto(named: name)
         }
     }