Fix an invalid link to pass the HTML Proofer

Author: byyue

_sips/minutes/2019-12-18-sip-minutes.md

@@ -161,7 +161,7 @@ New thread: [SIP public review: Open classes](https://contributors.scala-lang.or
 
 ### Review the "Explicit nulls" SIP
 
-Thread: <https://dotty.epfl.ch/docs/reference/other-new-features/explicit-nulls.html>
+Thread: <https://docs.scala-lang.org/scala3/reference/other-new-features/explicit-nulls.html>
 
 Sébastien championing and presenting.