fix the GRASS7 r_contour tool removing the mandatory flag of one of the parameters

Author: gioman

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

@@ -3,7 +3,7 @@ Produces a vector map of specified contours from a raster map.
 Raster (r.*)
 QgsProcessingParameterRasterLayer|input|Input raster|None|False
 QgsProcessingParameterNumber|step|Increment between contour levels|QgsProcessingParameterNumber.Double|None|True|None|None
-QgsProcessingParameterString|levels|List of contour levels|None|False
+QgsProcessingParameterString|levels|List of contour levels|None|False|True
 QgsProcessingParameterNumber|minlevel|Minimum contour level|QgsProcessingParameterNumber.Double|None|True|None|None
 QgsProcessingParameterNumber|maxlevel|Maximum contour level|QgsProcessingParameterNumber.Double|None|True|None|None
 QgsProcessingParameterNumber|cut|Minimum number of points for a contour line (0 -> no limit)|QgsProcessingParameterNumber.Integer|0|True|0|None