Fix - Documentation on isExpired() function

[altan-me]

https://github.com/salesforce-marketingcloud/FuelSDK-Node-Auth/wiki/isExpired
Github Wiki isExpired() true & false need to be swapped around..

Looking at the function the documentation should convey that:
if isExpired() returns true = token is expired or blank.
if isExpired() returns false = token is still valid