security/Kconfig: Correct the Documentation reference for PTI

When the config option for PTI was added a reference to documentation was
added as well. But the documentation did not exist at that point. The final
documentation has a different file name.

Fix it up to point to the proper file.

Fixes: 385ce0ea ("x86/mm/pti: Add Kconfig")
Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-mm@kvack.org
Cc: linux-security-module@vger.kernel.org
Cc: James Morris <james.l.morris@oracle.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/3009cc8ccbddcd897ec1e0cb6dda524929de0d14.1515799398.git.wking@tremily.us