fix another strong mode warning

pkgs/markdown/lib/src/block_parser.dart

diff --git a/pkgs/markdown/lib/src/block_parser.dart b/pkgs/markdown/lib/src/block_parser.dart
index 6e1cf9f..bc53d03 100644
--- a/pkgs/markdown/lib/src/block_parser.dart
+++ b/pkgs/markdown/lib/src/block_parser.dart
@@ -598,7 +598,7 @@
     }
 
     endItem();
-    var itemNodes = <Node>[];
+    var itemNodes = <Element>[];
 
     var anyEmptyLines = removeTrailingEmptyLines(items);
     var anyEmptyLinesBetweenBlocks = false;