Bump flutter_markdown version to latest (#2339)

fixes: https://github.com/flutter/samples/issues/2332

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experimental/linting_tool/pubspec.yaml

@@ -15,7 +15,7 @@ dependencies:
   cupertino_icons: ^1.0.2
   equatable: ^2.0.3
   file_selector: ^1.0.0
-  flutter_markdown: ^0.7.0
+  flutter_markdown: ^0.7.3
   google_fonts: ^6.0.0
   hive: ^2.0.4
   hive_flutter: ^1.1.0