Update sbt-mdoc to 2.2.1

Author: scala-steward

project/plugins.sbt

@@ -7,4 +7,4 @@ addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.16")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" %  "2.4.0")
 addSbtPlugin("org.xerial.sbt" % "sbt-sonatype" % "3.9.2")
 addSbtPlugin("com.jsuereth" % "sbt-pgp" % "2.0.1")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.2.0" )
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.2.1" )