r.fillnulls: sync second keyword with r.fill.stats

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@72376 15284696-431f-4ddb-bdfa-cd5b030d7da7

scripts/r.fillnulls/r.fillnulls.py

@@ -26,6 +26,7 @@
 #%module
 #% description: Fills no-data areas in raster maps using spline interpolation.
 #% keyword: raster
+#% keyword: surface
 #% keyword: elevation
 #% keyword: interpolation
 #%end