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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/locking/futex-requeue-pi.rst

···
 Requeueing of tasks from a non-PI futex to a PI futex requires
 special handling in order to ensure the underlying rt_mutex is never
 left without an owner if it has waiters; doing so would break the PI
-boosting logic [see rt-mutex-desgin.txt] For the purposes of
+boosting logic [see rt-mutex-design.rst] For the purposes of
 brevity, this action will be referred to as "requeue_pi" throughout
 this document.  Priority inheritance is abbreviated throughout as
 "PI".

Documentation/locking/ww-mutex-design.rst

···
 Wound/Wait Deadlock-Proof Mutex Design
 ======================================
 
-Please read mutex-design.txt first, as it applies to wait/wound mutexes too.
+Please read mutex-design.rst first, as it applies to wait/wound mutexes too.
 
 Motivation for WW-Mutexes
 -------------------------