[markdown] fix new lint RE await in an async function (#2416)

pkgs/markdown/tool/update_shared.dart

diff --git a/pkgs/markdown/tool/update_shared.dart b/pkgs/markdown/tool/update_shared.dart
index 6992cd5..d390213 100644
--- a/pkgs/markdown/tool/update_shared.dart
+++ b/pkgs/markdown/tool/update_shared.dart
@@ -11,7 +11,7 @@
     final request = await client.getUrl(Uri.parse(uri));
     final response = await request.close();
 
-    return response
+    return await response
         .transform(utf8.decoder)
         .transform(const JsonDecoder())
         .single;