Update sbt-scalafmt to 2.5.6 (#613)

Author: scala-steward

project/plugins.sbt

@@ -2,6 +2,6 @@ addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.14.4")
 addSbtPlugin("com.github.sbt" % "sbt-release" % "1.4.0")
 addSbtPlugin("com.github.sbt" % "sbt-pgp" % "2.3.1")
 addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.8.0")
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.5")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.6")
 addSbtPlugin("org.scoverage" % "sbt-coveralls" % "1.3.15")
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.4.0")