Update sbt-mdoc to 2.8.1 (#385)

Author: softwaremill-ci

project/plugins.sbt

@@ -1,5 +1,5 @@
 val sbtSoftwareMillVersion = "2.1.1"
 addSbtPlugin("com.softwaremill.sbt-softwaremill" % "sbt-softwaremill-common" % sbtSoftwareMillVersion)
 addSbtPlugin("com.softwaremill.sbt-softwaremill" % "sbt-softwaremill-publish" % sbtSoftwareMillVersion)
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.8.0")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.8.1")
 addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "1.1.4")