x86/pti: Document fix wrong index

In section <2. Runtime Cost>, fix wrong index.
Signed-off-by: Nzhenwei.pi <zhenwei.pi@youruncloud.com>
Signed-off-by: NThomas 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

Documentation/x86/pti.txt

@@ -78,7 +78,7 @@ this protection comes at a cost:
      non-PTI SYSCALL entry code, so requires mapping fewer
      things into the userspace page tables.  The downside is
      that stacks must be switched at entry time.
-  d. Global pages are disabled for all kernel structures not
+  c. Global pages are disabled for all kernel structures not
      mapped into both kernel and userspace page tables.  This
      feature of the MMU allows different processes to share TLB
      entries mapping the kernel.  Losing the feature means more