Update sbt-mdoc to 2.2.20 (zio#356)

Author: scala-steward

project/plugins.sbt

@@ -4,6 +4,6 @@ addSbtPlugin("com.eed3si9n"                      % "sbt-buildinfo"    % "0.10.0"
 addSbtPlugin("com.geirsson"                      % "sbt-ci-release"   % "1.5.7")
 addSbtPlugin("com.thoughtworks.sbt-api-mappings" % "sbt-api-mappings" % "3.0.0")
 addSbtPlugin("io.github.davidgregory084"         % "sbt-tpolecat"     % "0.1.17")
-addSbtPlugin("org.scalameta"                     % "sbt-mdoc"         % "2.2.19")
+addSbtPlugin("org.scalameta"                     % "sbt-mdoc"         % "2.2.20")
 addSbtPlugin("org.scalameta"                     % "sbt-scalafmt"     % "2.4.2")
 addSbtPlugin("pl.project13.scala"                % "sbt-jmh"          % "0.4.0")