docs: networking: phy: Improve phrasing

It's not about times (multiple occurences of an event) but about the
duration of a time interval.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>