fix(admin): fix heading underline length after dashboard copy-paste fix

Fixing the copy-paste error ("activity app" → "dashboard app") made the
heading longer than its underline, causing a build warning.

AI-Assisted-By: claude-sonnet-4-6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala

admin_manual/configuration_server/dashboard_configuration.rst

@@ -11,7 +11,7 @@ The Dashboard App is shipped and enabled by default. If it is not enabled
 simply go to your Nextcloud Apps page to enable it.
 
 Configuring your Nextcloud for the dashboard app
------------------------------------------------
+-------------------------------------------------
 
 The dashboard widgets are provided by apps and have a unique identifier. This can be used to customize the default layout of the dashboard as an administrator. The layout is stored as a comma-separated list of widget identifiers.