Merge pull request #3585 from scala-steward/update/mdoc-2.6.4

Update mdoc, sbt-mdoc to 2.6.4

Author: mergify[bot]

project/plugins.sbt

@@ -9,7 +9,7 @@ addSbtPlugin("de.heikoseeberger" % "sbt-header" % "5.10.0")
 addSbtPlugin("org.typelevel" % "sbt-tpolecat" % "0.5.2")
 addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.4.7")
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.3.2")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.6.3")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.6.4")
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.3.0")
 addSbtPlugin("org.typelevel" % "sbt-typelevel-mergify" % "0.7.7")