Update sbt-dotty to 0.4.6 (scala#463)

Author: scala-steward

project/plugins.sbt

@@ -4,4 +4,4 @@ val scalaJSVersion =
 addSbtPlugin("org.scala-lang.modules" % "sbt-scala-module" % "2.2.3")
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.0.0")
 addSbtPlugin("org.scala-js" % "sbt-scalajs" % scalaJSVersion)
-addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.4.5")
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.4.6")