Update sbt-mdoc to 2.6.1

Author: scala-steward

project/plugins.sbt

@@ -2,5 +2,5 @@ addSbtPlugin("org.scalameta"    % "sbt-scalafmt"   % "2.5.2")
 addSbtPlugin("ch.epfl.scala"    % "sbt-scalafix"   % "0.11.1")
 addSbtPlugin("com.github.sbt"   % "sbt-jacoco"     % "3.5.0")
 addSbtPlugin("com.timushev.sbt" % "sbt-updates"    % "0.6.4")
-addSbtPlugin("org.scalameta"    % "sbt-mdoc"       % "2.6.0")
+addSbtPlugin("org.scalameta"    % "sbt-mdoc"       % "2.6.1")
 addSbtPlugin("com.github.sbt"   % "sbt-ci-release" % "1.6.1")