docs: fix Sphinx toctree warning.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170813 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/index.rst b/docs/index.rst
index 1c223ef..e525a1d 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -31,6 +31,7 @@
    AutomaticReferenceCounting
    InternalsManual
    DriverInternals
+   BlockLanguageSpec
 
 Indices and tables
 ==================