fix missing underline

source/guides/appointments-external-scheduler.rst

@@ -50,7 +50,7 @@ the dates for appointments for study sessions themselves.
 .. _make-appointments-with-external-scheduler:
 
 Make appointments with the external scheduler
-============================================
+=============================================
 
 Castellum allows the use of an external scheduler. That way subjects can select
 the dates for appointments for study sessions themselves.