x86/pti: Document fix wrong index

commit 98f0fceec7f84d80bc053e49e596088573086421 upstream.

In section <2. Runtime Cost>, fix wrong index.

Signed-off-by: zhenwei.pi <zhenwei.pi@youruncloud.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: dave.hansen@linux.intel.com
Link: https://lkml.kernel.org/r/1516237492-27739-1-git-send-email-zhenwei.pi@youruncloud.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>