Fix wrong identifier name in Documentation/kref.txt

There's a typo / wrong identifier name in Documentation/kref.txt. Fix it.

Signed-off-by: Satyam Sharma <ssatyam@cse.iitk.ac.in>
Acked-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>