Update sbt-scalafmt to 2.5.6 (#398)

Author: scala-steward

project/plugins.sbt

@@ -3,5 +3,5 @@ addSbtPlugin("org.portable-scala" % "sbt-crossproject" % "1.3.2")
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.3.2")
 addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "1.3.2")
 addSbtPlugin("org.scala-native" % "sbt-scala-native" % "0.5.9")
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.5")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.6")
 addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.11.2")