[markdown, stream_transform] Fix deprecated usage of pkg:web (#2046)

Author: kevmoo

pkgs/stream_transform/example/main.dart

@@ -16,7 +16,7 @@ void main() {
       .tap((v) {
     print('Saw: $v');
   }).forEach((v) {
-    output.text = v;
+    output.textContent = v;
   });
 }