fix epilog setter

Author: blink1073

jupyter_core/command.py

@@ -30,7 +30,7 @@ def epilog(self):
         return "Available subcommands: %s" % " ".join(list_subcommands())
 
     @epilog.setter
-    def epilog(self):
+    def epilog(self, x):
         """Ignore epilog set in Parser.__init__"""
         pass