fix an expansion that doesn't work in ash

src/main/resources/com/typesafe/sbt/packager/archetypes/scripts/ash-template

@@ -94,7 +94,7 @@ process_args () {
      -java-home) require_arg path "$1" "$2" && jre=`eval echo $2` && java_cmd="$jre/bin/java" && shift 2 ;;
 
  -D*|-agentlib*|-agentpath*|-javaagent*|-XX*) addJava "$1" && shift ;;
-                                         -J*) addJava "${1:2}" && shift ;;
+                                         -J*) addJava "$(printf %s "$1"  | sed s/^..//)" && shift ;;
                                            *) addResidual "$1" && shift ;;
     esac
   done