[observe] use consistent comment style

pkg/observe/lib/src/observable_box.dart

Index: pkg/observe/lib/src/observable_box.dart
diff --git a/pkg/observe/lib/src/observable_box.dart b/pkg/observe/lib/src/observable_box.dart
index 9455dbd17ffbc555f5585dc22ef842c3da03adb0..dfaf7ffb5960607615aa851acea920d9a125115c 100644
--- a/pkg/observe/lib/src/observable_box.dart
+++ b/pkg/observe/lib/src/observable_box.dart
@@ -8,12 +8,10 @@ import 'package:observe/observe.dart';
 
 // TODO(jmesserly): should the property name be configurable?
 // That would be more convenient.
-/**
- * An observable box that holds a value. Use this if you want to store a single
- * value. For other cases, it is better to use [ObservableList],
- * [ObservableMap], or a custom [Observable] implementation based on
- * [Observable]. The property name for changes is "value".
- */
+/// An observable box that holds a value. Use this if you want to store a single
+/// value. For other cases, it is better to use [ObservableList],
+/// [ObservableMap], or a custom [Observable] implementation based on
+/// [Observable]. The property name for changes is "value".
 class ObservableBox<T> extends ChangeNotifier {
   T _value;