Update sbt-scalafmt to 2.5.6 (#435)

Author: scala-steward

project/plugins.sbt

@@ -8,7 +8,7 @@ addSbtPlugin("net.virtual-void" % "sbt-dependency-graph" % "0.10.0-RC1")
 
 addSbtPlugin("cchantep" % "sbt-scaladoc-compiler" % "0.3")
 
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.5")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.6")
 
 addSbtPlugin("cchantep" % "sbt-hl-compiler" % "0.9")