timekeeping.txt: Correct maxCount of n-bit binary counter

A n-bit binary counter can count a maximum of 2^n events and the count
value ranges from 0 to (2^n)-1

Signed-off-by: Siddaraju DH <siddarajudh@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
1 file changed