Update sbt-scalafmt to 2.5.6 (#993)

Author: scala-steward

project/plugins.sbt

@@ -1,6 +1,6 @@
 addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.11.2")
 addSbtPlugin("com.eed3si9n"   % "sbt-assembly"   % "2.3.1")
-addSbtPlugin("org.scalameta"  % "sbt-scalafmt"   % "2.5.5")
+addSbtPlugin("org.scalameta"  % "sbt-scalafmt"   % "2.5.6")
 addSbtPlugin("org.scoverage"  % "sbt-scoverage"  % "2.4.0")
 addSbtPlugin("ch.epfl.scala"  % "sbt-scalafix"   % "0.14.4")