Fix typo in docs

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294115 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/DesignDocs/DebugMode.rst b/docs/DesignDocs/DebugMode.rst
index 166c733..3b997d4 100644
--- a/docs/DesignDocs/DebugMode.rst
+++ b/docs/DesignDocs/DebugMode.rst
@@ -3,7 +3,7 @@
 ==========
 
 .. contents::
-   :local
+   :local:
 
 .. _using-debug-mode: