Update sbt-mdoc to 2.2.20

Author: scala-steward

project/plugins.sbt

@@ -6,5 +6,5 @@ addSbtPlugin("com.github.tkawachi" % "sbt-doctest" % "0.9.9")
 addSbtPlugin("com.typesafe.sbt" % "sbt-osgi" % "0.9.6")
 addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.4.0")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.4.2")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.2.19")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.2.20")
 addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.27")