futex: MAINTAINERS: Re-add selftests directory

Commit 95ca6d73 ("docs: move locking-specific documents to
locking/") accidentally replaced the selftests line for a duplicated
documentation one. Revert this change.
Signed-off-by: André Almeida <andrealmeid@collabora.com>
Fixes: 95ca6d73 ("docs: move locking-specific documents to locking/")
Link: https://lore.kernel.org/r/20200715131036.9692-1-andrealmeid@collabora.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

MAINTAINERS

@@ -7106,7 +7106,7 @@ F:	include/linux/futex.h
 F:	include/uapi/linux/futex.h
 F:	kernel/futex.c
 F:	tools/perf/bench/futex*
-F:	Documentation/locking/*futex*
+F:	tools/testing/selftests/futex/
 
 GATEWORKS SYSTEM CONTROLLER (GSC) DRIVER
 M:	Tim Harvey <tharvey@gateworks.com>