Update sbt-scalafmt to 2.5.6 (#1919)

Author: scala-steward

project/plugins.sbt

@@ -1,6 +1,6 @@
 addSbtPlugin("io.github.davidgregory084" % "sbt-tpolecat"              % "0.1.22")
 addSbtPlugin("io.spray"                  % "sbt-revolver"              % "0.10.0")
-addSbtPlugin("org.scalameta"             % "sbt-scalafmt"              % "2.5.4")
+addSbtPlugin("org.scalameta"             % "sbt-scalafmt"              % "2.5.6")
 addSbtPlugin("com.sksamuel.scapegoat"    %% "sbt-scapegoat"            % "1.2.13")
 addSbtPlugin("ch.epfl.scala"             % "sbt-scalafix"              % "0.11.1")
 addSbtPlugin("com.eed3si9n"              % "sbt-assembly"              % "0.14.9")