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

pkg/csslib/lib/parser.dart

Index: pkg/csslib/lib/parser.dart
diff --git a/pkg/csslib/lib/parser.dart b/pkg/csslib/lib/parser.dart
index ba8cd1360cf3b7ba4079f6d6b8021c2724ab2580..2539d94de14da2dbded62a2b7ea95a1e943ef12e 100644
--- a/pkg/csslib/lib/parser.dart
+++ b/pkg/csslib/lib/parser.dart
@@ -1002,7 +1002,7 @@ class _Parser {
    *      color: red;
    *    }
    *
-   * Return [null] if no selector or [SelectorGroup] if a selector was parsed.
+   * Return [:null:] if no selector or [SelectorGroup] if a selector was parsed.
    */
   SelectorGroup _nestedSelector() {
     Messages oldMessages = messages;