Update sbt-scalafmt to 2.5.6

project/plugins.sbt

diff --git a/project/plugins.sbt b/project/plugins.sbt
index fa99da0..1532a7c 100644
--- a/project/plugins.sbt
+++ b/project/plugins.sbt
@@ -25,4 +25,4 @@
 
 addSbtPlugin("com.github.sbt" % "sbt-unidoc" % "0.6.1")
 
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.4")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.6")