Update mdoc, sbt-mdoc to 2.9.1 (#1206)

Co-authored-by: zio-scala-steward[bot] <145262613+zio-scala-steward[bot]@users.noreply.github.com>
Co-authored-by: Jules Ivanic <jules.ivanic@gmail.com>

Author: zio-scala-steward[bot]

project/plugins.sbt

@@ -4,7 +4,7 @@ addSbtPlugin("com.thoughtworks.sbt-api-mappings" % "sbt-api-mappings"          %
 addSbtPlugin("com.github.sbt"                    % "sbt-unidoc"                % "0.6.1")
 addSbtPlugin("org.typelevel"                     % "sbt-tpolecat"              % "0.5.7")
 addSbtPlugin("org.scalameta"                     % "sbt-scalafmt"              % "2.6.2")
-addSbtPlugin("org.scalameta"                     % "sbt-mdoc"                  % "2.9.0")
+addSbtPlugin("org.scalameta"                     % "sbt-mdoc"                  % "2.9.1")
 addSbtPlugin("org.scoverage"                     % "sbt-scoverage"             % "2.4.4")
 addSbtPlugin("dev.zio"                           % "zio-sbt-ci"                % zioSbtVersion)
 addSbtPlugin("dev.zio"                           % "zio-sbt-ecosystem"         % zioSbtVersion)