Update bymonthdays in update request (#175)

Should have been updated together with the create request but was
forgotten.

Author: Marenz

proto/frequenz/api/dispatch/v1/dispatch.proto

@@ -380,8 +380,11 @@ message UpdateMicrogridDispatchRequest {
       // On which day(s) of the week does the event occur
       repeated RecurrenceRule.Weekday byweekdays = 6;
 
-      // On which day(s) of the month does the event occur
-      repeated uint32 bymonthdays = 7;
+      // On which day(s) of the month does the event occur. Valid values are 1 to 31 or -31 to -1.
+      //
+      // For example, -10 represents the tenth to the last day of the month.
+      // The bymonthdays rule part MUST NOT be specified when the FREQ rule part is set to WEEKLY.
+      repeated int32 bymonthdays = 7;
 
       // On which month(s) of the year does the event occur
       repeated uint32 bymonths = 8;