Fix some sphinx doc errors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352887 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index 85c04f1..63db11b 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -4640,6 +4640,8 @@
                           type: !3)
     !2 = !DILocalVariable(name: "y", scope: !5, file: !2, line: 7, type: !3)
 
+.. _DIExpression:
+
 DIExpression
 """"""""""""