perf tools: Fix syntax in documentation

Fix trivial syntax in perf-events user-space tools
documentation.

Signed-off-by: Zeev Tarantov <zeev.tarantov@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <12d7e64c0911081811i7e5b466cu6706ff6ab3e70db4@mail.gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>