Fix server method in DevEnum example in doc/data_types.rst

Fix Clock.mktime server method in documentation data_types.rst DevEnum example.

Without the fix, the failure is like this for a DeviceProxy client:

>>> dp.mktime((2021,   12,    2,   12,   56,   32,    3,  336,    0))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/dist-packages/tango/device_proxy.py", line 275, in f
    return dp.command_inout(name, *args, **kwds)
  File "/usr/local/lib/python3.7/dist-packages/tango/green.py", line 195, in greener
    return executor.run(fn, args, kwargs, wait=wait, timeout=timeout)
  File "/usr/local/lib/python3.7/dist-packages/tango/green.py", line 109, in run
    return fn(*args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/tango/connection.py", line 108, in __Connection__command_inout
    r = Connection.command_inout_raw(self, name, *args, **kwds)
  File "/usr/local/lib/python3.7/dist-packages/tango/connection.py", line 137, in __Connection__command_inout_raw
    return self.__command_inout(cmd_name, param)
PyTango.DevFailed: DevFailed[
DevError[
    desc = TypeError: Tuple or struct_time argument required
           
  origin = Traceback (most recent call last):
  File "/usr/local/lib/python3.7/dist-packages/tango/server.py", line 1142, in cmd
    return get_worker().execute(f, self, *args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/tango/green.py", line 98, in execute
    return fn(*args, **kwargs)
  File "/training/Clock/ClockDS.py", line 71, in mktime
    return time.mktime(tupl)
TypeError: Tuple or struct_time argument required

  reason = PyDs_PythonError
severity = ERR]

DevError[
    desc = Failed to execute command_inout on device test/nodb/clock, command mktime
  origin = Connection::command_inout()
  reason = API_CommandFailed
severity = ERR]
]

On the device side, it reports the command's input argument has type <class 'numpy.ndarray'>.