Update mdoc, sbt-mdoc to 2.2.20

Author: 47erbot

project/plugins.sbt

@@ -3,7 +3,7 @@ addSbtPlugin("org.portable-scala"   % "sbt-scalajs-crossproject" % "1.0.0")
 addSbtPlugin("org.scala-js"         % "sbt-scalajs"              % "1.5.1")
 addSbtPlugin("org.scalameta"        % "sbt-scalafmt"             % "2.4.2")
 addSbtPlugin("com.47deg"            % "sbt-microsites"           % "1.3.3")
-addSbtPlugin("org.scalameta"        % "sbt-mdoc"                 % "2.2.19")
+addSbtPlugin("org.scalameta"        % "sbt-mdoc"                 % "2.2.20")
 addSbtPlugin("de.heikoseeberger"    % "sbt-header"               % "5.6.0")
 addSbtPlugin("com.alejandrohdezma" %% "sbt-github"               % "0.9.2")
 addSbtPlugin("com.alejandrohdezma"  % "sbt-github-header"        % "0.9.2")