Update mdoc, sbt-mdoc to 2.6.4

Author: scala-steward

project/plugins.sbt

@@ -2,4 +2,4 @@ val sbtSoftwareMillVersion = "2.0.21"
 addSbtPlugin("com.softwaremill.sbt-softwaremill" % "sbt-softwaremill-common" % sbtSoftwareMillVersion)
 addSbtPlugin("com.softwaremill.sbt-softwaremill" % "sbt-softwaremill-publish" % sbtSoftwareMillVersion)
 addSbtPlugin("com.eed3si9n" % "sbt-projectmatrix" % "0.10.1")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.6.3")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.6.4")