Update sbt-mdoc to 2.1.3 (#369)

Author: scala-steward

project/plugins.sbt

@@ -2,5 +2,5 @@ addSbtPlugin("com.47deg"                 % "sbt-org-policies" % "0.13.1")
 addSbtPlugin("com.47deg"                 % "sbt-microsites"   % "1.1.3")
 addSbtPlugin("com.eed3si9n"              % "sbt-buildinfo"    % "0.9.0")
 addSbtPlugin("org.scoverage"             % "sbt-scoverage"    % "1.6.1")
-addSbtPlugin("org.scalameta"             % "sbt-mdoc"         % "2.1.1")
+addSbtPlugin("org.scalameta"             % "sbt-mdoc"         % "2.1.3")
 addSbtPlugin("io.github.davidgregory084" % "sbt-tpolecat"     % "0.1.11")