diff --git a/doc/manual/core_drivers_reference.rst b/doc/manual/core_drivers_reference.rst
index 66e3cfea6..08633b7c6 100644
--- a/doc/manual/core_drivers_reference.rst
+++ b/doc/manual/core_drivers_reference.rst
@@ -33,7 +33,7 @@ System drivers
 
 
 Digital I/O drivers
-+++++++++++++++++++
+-------------------
 
 :mod:`artiq.coredevice.ttl` module
 ++++++++++++++++++++++++++++++++++