Update sbt-mdoc to 2.3.1

Author: 47erbot

project/plugins.sbt

@@ -1,6 +1,6 @@
 addSbtPlugin("org.scala-exercises" % "sbt-exercise"             % "0.7.1")
 addSbtPlugin("com.github.sbt"      % "sbt-ci-release"           % "1.5.10")
-addSbtPlugin("org.scalameta"       % "sbt-mdoc"                 % "2.3.0")
+addSbtPlugin("org.scalameta"       % "sbt-mdoc"                 % "2.3.1")
 addSbtPlugin("org.scalameta"       % "sbt-scalafmt"             % "2.4.6")
 addSbtPlugin("de.heikoseeberger"   % "sbt-header"               % "5.6.5")
 addSbtPlugin("com.alejandrohdezma" % "sbt-github"               % "0.11.2")