fix URL in hpet.txt

That URL is out of date. Update it.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Matt LaPlante <kernel1@cyberdogtech.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: WANG Cong <amwang@redhat.com>
Acked-by: Matt LaPlante <kernel@cyberdogtech.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/timers/hpet.txt

@@ -3,7 +3,7 @@
 The High Precision Event Timer (HPET) hardware follows a specification
 by Intel and Microsoft which can be found at
 
-	http://www.intel.com/technology/architecture/hpetspec.htm
+	http://www.intel.com/hardwaredesign/hpetspec_1.pdf
 
 Each HPET has one fixed-rate counter (at 10+ MHz, hence "High Precision")
 and up to 32 comparators.  Normally three or more comparators are provided,