Fix command

biopet-extensions/src/main/scala/nl/lumc/sasc/biopet/extensions/centrifuge/Centrifuge.scala

@@ -44,7 +44,7 @@ class Centrifuge(val root: Configurable) extends BiopetCommandLineFunction with
    */
   def cmdLine: String = executable +
     //TODO: Options
-    optional("--threads", threads)
+    optional("--threads", threads) +
     required("-x", index) +
     (inputR2 match {
       case Some(r2) => required("-1", inputR1) + required("-2", r2)