Fix the level of a title in docs

Author: ShapelessCat

docs/blog/_posts/2021-02-17-scala3-rc1.md

@@ -153,7 +153,7 @@ If you have questions or any sort of feedback, feel free to send us a message on
 [Gitter channel](https://gitter.im/lampepfl/dotty). If you encounter a bug, please
 [open an issue on GitHub](https://github.com/lampepfl/dotty/issues/new).
 
-### Contributors
+## Contributors
 
 Thank you to all the contributors who made this release possible 🎉