d.wms: add png8 format option

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@57024 15284696-431f-4ddb-bdfa-cd5b030d7da7

gui/scripts/d.wms.py

@@ -58,7 +58,7 @@
 #% key: format
 #% type: string
 #% description: Image format requested from the server
-#% options: geotiff,tiff,jpeg,gif,png
+#% options: geotiff,tiff,jpeg,gif,png,png8
 #% answer: geotiff
 #% guisection: Request
 #%end