Update sbt-dotty to 0.5.1 (#333)

Author: scala-steward

project/plugins.sbt

@@ -12,4 +12,4 @@ addSbtPlugin("org.scala-js" % "sbt-scalajs" % scalaJSVersion)
 addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "1.0.0")
 addSbtPlugin("org.scala-native" % "sbt-scala-native" % scalaNativeVersion)
 
-addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.4.6")
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.5.1")