[the-book] Fix broken Hub link

Change-Id: Idfa1c78067d34f42f7515a8793b3b6c5412fdc2e
diff --git a/docs/the-book/namespaces.md b/docs/the-book/namespaces.md
index 145cafb..575aa6d 100644
--- a/docs/the-book/namespaces.md
+++ b/docs/the-book/namespaces.md
@@ -228,7 +228,7 @@
   - `vendor.topic.Interface`: service defined by a _vendor_
 - `dev/`: device tree (relevant portions visible to privileged components as needed)
   - `class/`, ...
-- `hub/`: introspect the system, see [hub.md] (privileged components only)
+- `hub/`: introspect the system, see [Hub](hub.md) (privileged components only)
 - `config/`: configuration data for the component
 
 ## Namespace Participants