Documentation: locking: fix references

This patch fixes file references from txt to rst file ending in
ww-mutex-design.rst and futex-requeue-pi.rst. While on it fix a spelling
issue "desgin" to "design" reported by Matthew Wilcox.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20210823183143.1691344-1-aahringo@redhat.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>