[docs] Fix Sphinx warning.

Adornments need to be at least as long as the thing they adorn.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190327 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index fe23ac0..98654f3 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -8697,7 +8697,7 @@
 calling the ``__stack_chk_fail()`` function.
 
 '``llvm.stackprotectorcheck``' Intrinsic
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 """""""