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: NSiddaraju DH <siddarajudh@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/timers/timekeeping.txt

@@ -27,7 +27,7 @@ a Linux system will eventually read the clock source to determine exactly
 what time it is.
 
 Typically the clock source is a monotonic, atomic counter which will provide
-n bits which count from 0 to 2^(n-1) and then wraps around to 0 and start over.
+n bits which count from 0 to (2^n)-1 and then wraps around to 0 and start over.
 It will ideally NEVER stop ticking as long as the system is running. It
 may stop during system suspend.