use proper notation for true, false, and null in doc comments

sdk/lib/_internal/lib/isolate_helper.dart

Index: sdk/lib/_internal/lib/isolate_helper.dart
diff --git a/sdk/lib/_internal/lib/isolate_helper.dart b/sdk/lib/_internal/lib/isolate_helper.dart
index af5c9d55185f39e4c96c4ea28a46acce19f6f987..da534821af6bda0c174c849cc37e01b719d2793a 100644
--- a/sdk/lib/_internal/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/lib/isolate_helper.dart
@@ -152,7 +152,7 @@ class _Manager {
   /**
    * Whether to use the web-worker JSON-based message serialization protocol. By
    * default this is only used with web workers. For debugging, you can force
-   * using this protocol by changing this field value to [true].
+   * using this protocol by changing this field value to [:true:].
    */
   bool get needSerialization => useWorkers;