Update sbt-unidoc to 0.6.0 (#196)

Author: scala-steward

project/plugins.sbt

@@ -14,6 +14,6 @@ addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.11.2")
 
 addSbtPlugin("com.github.sbt" % "sbt-ghpages" % "0.9.0")
 
-addSbtPlugin("com.github.sbt" % "sbt-unidoc" % "0.5.0")
+addSbtPlugin("com.github.sbt" % "sbt-unidoc" % "0.6.0")
 
 addSbtPlugin("com.thoughtworks.sbt-api-mappings" % "sbt-api-mappings" % "3.0.2")