Update sbt-doctest to 0.9.9 (#1895)

Author: scala-steward

project/plugins.sbt

@@ -4,7 +4,7 @@ addSbtPlugin("com.eed3si9n" % "sbt-buildinfo" % "0.10.0")
 addSbtPlugin("com.geirsson" % "sbt-ci-release" % "1.5.5")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.4.2")
 addSbtPlugin("com.github.cb372" % "sbt-explicit-dependencies" % "0.2.16")
-addSbtPlugin("com.github.tkawachi" % "sbt-doctest" % "0.9.8")
+addSbtPlugin("com.github.tkawachi" % "sbt-doctest" % "0.9.9")
 addSbtPlugin("com.typesafe.sbt" % "sbt-native-packager" % "1.8.0")
 addSbtPlugin("de.heikoseeberger" % "sbt-header" % "5.6.0")
 addSbtPlugin("io.github.davidgregory084" % "sbt-tpolecat" % "0.1.16")