Update sbt-mdoc to 2.2.2 (#201)

Author: scala-steward

project/plugins.sbt

@@ -1,6 +1,6 @@
 addSbtPlugin("org.scala-exercises" % "sbt-exercise"             % "0.6.1")
 addSbtPlugin("com.geirsson"        % "sbt-ci-release"           % "1.5.3")
-addSbtPlugin("org.scalameta"       % "sbt-mdoc"                 % "2.1.5")
+addSbtPlugin("org.scalameta"       % "sbt-mdoc"                 % "2.2.2")
 addSbtPlugin("org.scalameta"       % "sbt-scalafmt"             % "2.4.0")
 addSbtPlugin("de.heikoseeberger"   % "sbt-header"               % "5.6.0")
 addSbtPlugin("com.alejandrohdezma" % "sbt-github"               % "0.8.2")