Update explicit-nulls.md (#19327)

Fix link

Fixes [this](scala/docs.scala-lang#2941)
issue.

docs/_docs/reference/experimental/explicit-nulls.md

@@ -1,7 +1,7 @@
 ---
 layout: doc-page
 title: "Explicit Nulls"
-nightlyOf: https://docs.scala-lang.org/scala3/reference/other-new-features/explicit-nulls.html
+nightlyOf: https://docs.scala-lang.org/scala3/reference/experimental/explicit-nulls.html
 ---
 
 Explicit nulls is an opt-in feature that modifies the Scala type system, which makes reference types