[processing][grass] Fix broken r.his algorithm

Author: nyalldawson

python/plugins/processing/algs/grass7/description/r.his.txt

@@ -4,7 +4,7 @@ Raster (r.*)
 QgsProcessingParameterRasterLayer|hue|Hue|None|False
 QgsProcessingParameterRasterLayer|intensity|Intensity|None|False
 QgsProcessingParameterRasterLayer|saturation|Saturation|None|False
-QgsProcessingParameterString|bgcolor|Color to use instead of NULL values. Either a standard color name, R:G:B triplet, or "none"|'none'|False|True
+QgsProcessingParameterString|bgcolor|Color to use instead of NULL values. Either a standard color name, R:G:B triplet, or "none"|None|False|True
 QgsProcessingParameterBoolean|-c|Use colors from color tables for NULL values|False
 QgsProcessingParameterRasterDestination|red|Red
 QgsProcessingParameterRasterDestination|green|Green