Update sbt-mdoc to 2.2.9

Author: 47erbot

project/plugins.sbt

@@ -2,7 +2,7 @@ addSbtPlugin("com.geirsson"              % "sbt-ci-release"           % "1.5.3")
 addSbtPlugin("com.47deg"                 % "sbt-microsites"           % "1.2.1")
 addSbtPlugin("org.scoverage"             % "sbt-scoverage"            % "1.6.1")
 addSbtPlugin("org.scalameta"             % "sbt-scalafmt"             % "2.4.2")
-addSbtPlugin("org.scalameta"             % "sbt-mdoc"                 % "2.2.8")
+addSbtPlugin("org.scalameta"             % "sbt-mdoc"                 % "2.2.9")
 addSbtPlugin("com.eed3si9n"              % "sbt-unidoc"               % "0.4.3")
 addSbtPlugin("de.heikoseeberger"         % "sbt-header"               % "5.6.0")
 addSbtPlugin("com.alejandrohdezma"       % "sbt-codecov"              % "0.2.0")