Update sbt-scalafmt to 2.5.6 (#605)

Author: xerial-bot

project/plugins.sbt

@@ -4,6 +4,6 @@ addSbtPlugin("com.github.sbt"          % "sbt-pgp"      % "2.3.1")
 addSbtPlugin("org.scalameta"           % "sbt-scalafmt" % "2.3.4")
 addSbtPlugin("org.playframework.twirl" % "sbt-twirl"    % "2.0.9")
 addSbtPlugin("com.github.sbt"          % "sbt-dynver"   % "5.1.1")
-addSbtPlugin("org.scalameta"           % "sbt-scalafmt" % "2.5.5")
+addSbtPlugin("org.scalameta"           % "sbt-scalafmt" % "2.5.6")
 
 libraryDependencies += "org.scala-sbt" %% "scripted-plugin" % sbtVersion.value