support latest version of pkg/html

pkgs/markdown/pubspec.yaml

diff --git a/pkgs/markdown/pubspec.yaml b/pkgs/markdown/pubspec.yaml
index 48690dd..c84ba4d 100644
--- a/pkgs/markdown/pubspec.yaml
+++ b/pkgs/markdown/pubspec.yaml
@@ -10,7 +10,7 @@
   # Only for example/app.dart.
   browser: any
   collection: ^1.2.0
-  html: '^0.12.2'
+  html: '>=0.12.2 <0.14.0'
   path: '^1.3.1'
   test: '^0.12.4+1'
   yaml: '^2.1.8'