perf probe: Fix spelling mistake in documentation

Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1291168642-11402-8-git-send-email-shawn.bohrer@gmail.com>
Signed-off-by: Shawn Bohrer <shawn.bohrer@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>