Update mdoc, sbt-mdoc to 2.3.8 (#3187)

Author: scala-steward

project/plugins.sbt

@@ -9,6 +9,6 @@ addSbtPlugin("de.heikoseeberger" % "sbt-header" % "5.10.0")
 addSbtPlugin("org.typelevel" % "sbt-tpolecat" % "0.5.0")
 addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.4.6")
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.3.2")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.3.7")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.3.8")
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.0.9")
 addSbtPlugin("org.typelevel" % "sbt-typelevel-mergify" % "0.5.4")