Update sbt-scalafmt to 2.3.1

Author: scala-steward

project/plugins.sbt

@@ -15,4 +15,4 @@ addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "0.6.1")
 addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.3.7")
 addSbtPlugin("com.eed3si9n" % "sbt-buildinfo" % "0.9.0")
 addSbtPlugin("com.dwijnand" % "sbt-travisci" % "1.2.0")
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.3.0")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.3.1")