Update pytorch_lightning/callbacks/timer.py

Co-authored-by: Akihiro Nitta <nitta@akihironitta.com>

pytorch_lightning/callbacks/timer.py

@@ -44,7 +44,7 @@ class Timer(Callback):
  duration: A string in the format DD:HH:MM:SS (days, hours, minutes seconds), or a :class:`datetime.timedelta`,
  or a dict containing key-value compatible with :class:`~datetime.timedelta`.
  interval: Determines if the interruption happens on epoch level or mid-epoch.
- Can be either `epoch` or `step`.
+ Can be either ``"epoch"`` or ``"step"``.
  verbose: Set this to ``False`` to suppress logging messages.
 
  Raises: