Update sbt-scalafmt to 2.5.6 (#758)

Author: scala-steward

project/plugins.sbt

@@ -12,7 +12,7 @@ addSbtPlugin("com.github.sbt" % "sbt-dynver" % "5.1.1")
 
 addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.7.2")
 
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.5")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.6")
 
 addSbtPlugin("com.evolution" % "sbt-scalac-opts-plugin" % "0.0.9")