Documentation: locking: ww-mutex-design: drop duplicated word

Drop the doubled word "up".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200703213649.30948-3-rdunlap@infradead.org


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