Update sbt-scalafmt to 2.5.6 (#235)

Author: scala-steward

project/plugins.sbt

@@ -3,4 +3,4 @@ addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.11.2")
 addSbtPlugin("com.github.sbt" % "sbt-native-packager" % "1.11.4")
 addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "2.3.1")
 addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.14.4")
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.5")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.6")