docs: fix Sphinx toctree warning.

llvm-svn: 170813
diff --git a/clang/docs/index.rst b/clang/docs/index.rst
index 1c223ef..e525a1d 100644
--- a/clang/docs/index.rst
+++ b/clang/docs/index.rst
@@ -31,6 +31,7 @@
    AutomaticReferenceCounting
    InternalsManual
    DriverInternals
+   BlockLanguageSpec
 
 Indices and tables
 ==================