-skip-packages ... no longer works; use -skip-packages: (#28)

Running `sbt publishLocal` caused a build-failure on my end. This is
caused by the fact that the sbt instructions used a removed syntax for
scaladoc: ["-skip-packages", "..."] instead of "-skip-packages:...". See
scala/scala3#14939.

Now 'sbt +publishLocal' works without errors.

build.sbt

@@ -100,8 +100,7 @@ lazy val docSettings = Seq(
   Compile / doc / scalacOptions ++= Seq(
     // ANTLR-generated classes aren't really part of public API and cause
     // errors in ScalaDoc generation
-    "-skip-packages",
-    "firrtl2.antlr",
+    "-skip-packages:firrtl2.antlr",
     "-Xfatal-warnings",
     "-feature",
     "-diagrams",