Adjust the CacheFiles documentation to use the correct names of the credential
pointers in task_struct.

The documentation was using names from the old versions of the credentials
patches.
Signed-off-by: NMarc Dionne <marc.c.dionne@gmail.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>