PM: docs: Fix stale reference in kernel-parameters.txt

Commit 7aa7a0360a66 (PM: docs: Delete the obsolete states.txt
document) forgot to update kernel-parameters.txt with a reference
to the new sleep-states.rst document and it still points to
states.txt that was dropped, so fix it now.

Fixes: 7aa7a0360a66 (PM: docs: Delete the obsolete states.txt document)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -2248,7 +2248,7 @@
 			s2idle  - Suspend-To-Idle
 			shallow - Power-On Suspend or equivalent (if supported)
 			deep    - Suspend-To-RAM or equivalent (if supported)
-			See Documentation/power/states.txt.
+			See Documentation/admin-guide/pm/sleep-states.rst.
 
 	meye.*=		[HW] Set MotionEye Camera parameters
 			See Documentation/video4linux/meye.txt.