Documentation: PM: cpuidle: correct typo

cerainly -> certainly

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: JuliaLawall

Documentation/admin-guide/pm/cpuidle.rst

@@ -494,7 +494,7 @@ object corresponding to it, as follows:
 	residency.
 
 ``below``
-	Total number of times this idle state had been asked for, but cerainly
+	Total number of times this idle state had been asked for, but certainly
 	a deeper idle state would have been a better match for the observed idle
 	duration.