Fix rpw implementation

app.py

@@ -134,6 +134,8 @@ class WatermarkDetector(WatermarkBase):
 
         if self.seeding_scheme == "simple_1":
             self.min_prefix_len = 1
+        elif self.seeding_scheme == "rpw":
+            self.min_prefix_len = 2
         else:
             raise NotImplementedError(f"Unexpected seeding_scheme: {self.seeding_scheme}")