fix grass7 r.drain non mandatory parameter

Author: Giovanni Manghi

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

@@ -3,7 +3,7 @@ r.drain - Traces a flow through an elevation model on a raster map.
 Raster (r.*)
 ParameterRaster|input|Elevation|False
 ParameterString|start_coordinates|Map coordinates of starting point(s) (E,N)|(0,0)
-ParameterMultipleInput|start_points|Vector layer(s) containing starting point(s)|0|False
+ParameterMultipleInput|start_points|Vector layer(s) containing starting point(s)|0|True
 ParameterBoolean|-c|Copy input cell values on output|False
 ParameterBoolean|-a|Accumulate input values along the path|False
 ParameterBoolean|-n|Count cell numbers along the path|False