Specs: clean up the last remaining mentions of documents

sky/specs/gestures.md

Index: sky/specs/gestures.md
diff --git a/sky/specs/gestures.md b/sky/specs/gestures.md
index e1773f579804fd09b24aabbd364426f50b408f15..f67559dd72d01721c48c32e66ebd3f530bd66327 100644
--- a/sky/specs/gestures.md
+++ b/sky/specs/gestures.md
@@ -170,7 +170,7 @@ class GestureManager {
   GestureManager(this.target) {
     target.events.where((event) => event is PointerDownEvent).listen(_handler);
   }
-  final @nonnull EventTarget target; // usually the ApplicationDocument object
+  final @nonnull EventTarget target; // usually the ApplicationRoot object
 
   Map<@nonnull int, @nonnull PointerState> _pointers = new SplayTreeMap<int, PointerState>();