Update sbt-mdoc to 2.6.4 (#247)

Author: scala-steward

project/plugin.sbt

@@ -1,6 +1,6 @@
 addSbtPlugin("org.scalameta"  % "sbt-scalafmt"   % "2.5.4")
 addSbtPlugin("ch.epfl.scala"  % "sbt-scalafix"   % "0.14.0")
 addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.9.2")
-addSbtPlugin("org.scalameta"  % "sbt-mdoc"       % "2.6.3")
+addSbtPlugin("org.scalameta"  % "sbt-mdoc"       % "2.6.4")
 addSbtPlugin("com.github.sbt" % "sbt-unidoc"     % "0.5.0")
 addSbtPlugin("com.eed3si9n"   % "sbt-buildinfo"  % "0.13.1")