fs: fix the location of the kernel-api book

The kernel-api book is now part of the core-api. Update its
location.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

fs/debugfs/inode.c

@@ -9,7 +9,7 @@
  *	2 as published by the Free Software Foundation.
  *
  *  debugfs is for people to use instead of /proc or /sys.
- *  See Documentation/DocBook/kernel-api for more details.
+ *  See ./Documentation/core-api/kernel-api.rst for more details.
  *
  */