perf bench futex: Remove bogus backslash from comment

It obviously doesn't belong there.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lore.kernel.org/lkml/20210809043301.66002-3-dave@stgolabs.net


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>