Fix heading format

Author: akash-sr

fern/docs/pages/customization.mdx

@@ -388,7 +388,7 @@ The object now references the latest fragment version (`custom_type_fragment/2`)
 the optional release notes field is absent, the tenant fragment remains attached,
 allowing for future tenant-specific field additions.
 
-## Bulk Upgrades
+#### Bulk Upgrades
 
 All objects of a given type can be upgraded to the latest fragment version using the 
 `objects.bulk-upgrade` API.