Update showCupertinoModalPopup documentation (#106274)

packages/flutter/lib/src/cupertino/route.dart

@@ -1135,7 +1135,7 @@ class CupertinoModalPopupRoute<T> extends PopupRoute<T> {
 ///
 /// The `useRootNavigator` argument is used to determine whether to push the
 /// popup to the [Navigator] furthest from or nearest to the given `context`. It
-/// is `false` by default.
+/// is `true` by default.
 ///
 /// The `semanticsDismissible` argument is used to determine whether the
 /// semantics of the modal barrier are included in the semantics tree.