Update sbt-scalafmt to 2.5.6 (#268)

Author: scala-steward

project/plugins.sbt

@@ -10,5 +10,5 @@ addSbtPlugin("org.xerial.sbt" % "sbt-sonatype" % "3.12.2")
 addSbtPlugin("com.eed3si9n" % "sbt-buildinfo" % "0.13.1")
 addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "2.3.1")
 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")
 addSbtPlugin("com.github.sbt" % "sbt-header" % "5.11.0")