Update sbt-mdoc to 2.2.0 (#490)

Author: scala-steward

project/plugins.sbt

@@ -3,7 +3,7 @@ addSbtPlugin("com.geirsson"              % "sbt-ci-release"    % "1.5.3")
 addSbtPlugin("com.47deg"                 % "sbt-microsites"    % "1.2.0")
 addSbtPlugin("org.scoverage"             % "sbt-scoverage"     % "1.6.1")
 addSbtPlugin("org.scalameta"             % "sbt-scalafmt"      % "2.3.3")
-addSbtPlugin("org.scalameta"             % "sbt-mdoc"          % "2.1.5")
+addSbtPlugin("org.scalameta"             % "sbt-mdoc"          % "2.2.0")
 addSbtPlugin("com.eed3si9n"              % "sbt-unidoc"        % "0.4.3")
 addSbtPlugin("de.heikoseeberger"         % "sbt-header"        % "5.4.0")
 addSbtPlugin("com.alejandrohdezma"      %% "sbt-github"        % "0.6.0")