Upgrade sbt-scalafmt from 2.5.5 to 2.5.6 (#263)

Author: scala-steward

project/plugins.sbt

@@ -4,4 +4,4 @@ addSbtPlugin("com.github.sbt" % "sbt-site"       % "1.6.0")
 addSbtPlugin("com.github.sbt" % "sbt-release"    % "1.4.0")
 addSbtPlugin("com.github.sbt" % "sbt-ghpages"    % "0.9.0")
 addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.11.2")
-addSbtPlugin("org.scalameta"  % "sbt-scalafmt"   % "2.5.5")
+addSbtPlugin("org.scalameta"  % "sbt-scalafmt"   % "2.5.6")