Fix an error message in python.el

* lisp/progmodes/python.el (python-shell-get-process-or-error):
Don't repeat the same key binding twice.  (Bug#25405)

lisp/progmodes/python.el

@@ -2826,8 +2826,7 @@ of `error' with a user-friendly message."
   (or (python-shell-get-process)
       (if interactivep
           (user-error
-           "Start a Python process first with `%s' or `%s'."
-           (substitute-command-keys "\\[run-python]")
+           "Start a Python process first with `M-x run-python' or `%s'."
            ;; Get the binding.
            (key-description
             (where-is-internal