Documentation: fix typo and update version in cachetlb.txt

Fix a typo, and the reference to a proposed v2.7 kernel, as
that in itself is dated.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>