Update sbt-scalafmt to 2.5.6 (#866)

Co-authored-by: scala-steward-asf[bot] <147768647+scala-steward-asf[bot]@users.noreply.github.com>

Author: scala-steward-asf[bot]

project/plugins.sbt

@@ -11,7 +11,7 @@ resolvers += Classpaths.sbtPluginReleases
 
 addSbtPlugin("com.github.sbt" % "sbt-multi-jvm" % "0.6.0")
 addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "1.1.4")
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.5")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.6")
 addSbtPlugin("com.github.sbt" % "sbt-dynver" % "5.1.1")
 addSbtPlugin("com.github.sbt" % "sbt-unidoc" % "0.6.0")
 addSbtPlugin("com.thoughtworks.sbt-api-mappings" % "sbt-api-mappings" % "3.0.2")