Update sbt-mdoc to 2.9.1

Author: typelevel-steward[bot]

project/plugins.sbt

@@ -3,5 +3,5 @@ addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.11.2")
 addSbtPlugin("com.github.sbt" % "sbt-unidoc" % "0.6.1")
 addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "1.1.5")
 addSbtPlugin("com.github.sbt" % "sbt-header" % "5.11.0")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.9.0")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.9.1")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.6.1")