Don't make retraction distance negative

Apparently this got changed accidentally because of a merge in an unfortunate time.

plugins/PostProcessingPlugin/scripts/FilamentChange.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2018 Ultimaker B.V.
+# Copyright (c) 2019 Ultimaker B.V.
 # The PostProcessingPlugin is released under the terms of the AGPLv3 or higher.
 
 from typing import Optional, Tuple
@@ -80,7 +80,7 @@ class FilamentChange(Script):
             color_change = color_change + (" E%.2f" % initial_retract)
 
         if later_retract is not None and later_retract > 0.:
-            color_change = color_change + (" L-%.2f" % later_retract)
+            color_change = color_change + (" L%.2f" % later_retract)
 
         if x_pos is not None:
             color_change = color_change + (" X%.2f" % x_pos)