Update sbt-mdoc to 2.2.8 (#271)

Author: scala-steward

project/plugins.sbt

@@ -3,7 +3,7 @@ addSbtPlugin("ch.epfl.scala"                     % "sbt-scalafix"     % "0.9.20"
 addSbtPlugin("pl.project13.scala"                % "sbt-jmh"          % "0.4.0")
 addSbtPlugin("com.eed3si9n"                      % "sbt-buildinfo"    % "0.10.0")
 addSbtPlugin("org.scoverage"                     % "sbt-scoverage"    % "1.6.1")
-addSbtPlugin("org.scalameta"                     % "sbt-mdoc"         % "2.2.7")
+addSbtPlugin("org.scalameta"                     % "sbt-mdoc"         % "2.2.8")
 addSbtPlugin("ch.epfl.scala"                     % "sbt-bloop"        % "1.4.4")
 addSbtPlugin("com.eed3si9n"                      % "sbt-unidoc"       % "0.4.3")
 addSbtPlugin("com.geirsson"                      % "sbt-ci-release"   % "1.5.3")