build(deps): bump sbt-scalafmt from 2.5.4 to 2.5.6 (#640)

Co-authored-by: Takeru Sato <type.in.type@gmail.com>

Author: scala-steward

project/plugins.sbt

@@ -1,4 +1,4 @@
-addSbtPlugin("org.scalameta"  % "sbt-scalafmt"   % "2.5.4")
+addSbtPlugin("org.scalameta"  % "sbt-scalafmt"   % "2.5.6")
 addSbtPlugin("org.scoverage"  % "sbt-scoverage"  % "2.4.1")
 addSbtPlugin("ch.epfl.scala"  % "sbt-scalafix"   % "0.14.3")
 addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.11.1")