Fix docs in FutureGroup (#171)

There is no `stream` member to reference.
diff --git a/lib/src/future_group.dart b/lib/src/future_group.dart
index c98fbc3..daf985d 100644
--- a/lib/src/future_group.dart
+++ b/lib/src/future_group.dart
@@ -51,12 +51,8 @@
   /// This stream will close when this group is idle *and* [close] has been
   /// called.
   ///
-  /// Note that:
-  ///
-  /// * Events won't be emitted on this stream until [stream] has been listened
-  ///   to.
-  /// * Events are delivered asynchronously, so it's possible for the group to
-  ///   become active again before the event is delivered.
+  /// Events are delivered asynchronously, so it's possible for the group to
+  /// become active again before the event is delivered.
   Stream get onIdle =>
       (_onIdleController ??= StreamController.broadcast(sync: true)).stream;