Update sbt-scalafmt to 2.5.6 (#218)

Author: scala-steward

project/plugins.sbt

@@ -2,7 +2,7 @@ addSbtPlugin("com.github.sbt" % "sbt-jni" % "1.7.0")
 
 addSbtPlugin("com.github.sbt" % "sbt-java-formatter" % "0.9.0")
 
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.5")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.6")
 
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.4.1")