[NFX] Fix language reference title declaration

llvm-svn: 351644
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 5600d2e..0843bf5 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -5067,7 +5067,7 @@
     !0 = !{i64 (i64, i64)* @add, i64 (i64, i64)* @sub}
 
 '``callback``' Metadata
-^^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^^
 
 ``callback`` metadata may be attached to a function declaration, or definition.
 (Call sites are excluded only due to the lack of a use case.) For ease of