Corrected error message

src/command_prompt.py

@@ -393,7 +393,7 @@ class Command_prompt(Label):
         elif argv[2]=="2square":
             self.app.painter.shape=Square2
         else:
-            self.message="error: unrecognized shape '"+argv[2]+"'; supported shapes are cross|disk"
+            self.message="error: unrecognized shape '"+argv[2]+"'; supported shapes are cross|disk|staircase|2square"
             return
 
     # toggle grid