To block access specifically to download.com while allowing other sites in the "Freeware and Software Downloads" category, you can create a separate firewall policy with a deny action specifically for the FQDN
*.download.com. This approach allows blocking this particular site without affecting the other sites in the same category. Alternatively, configuring a static URL filter entry with the type set to Wildcard and action set to Block will also achieve the desired effect by directly blocking the specific URL without impacting other sites in the category.
References:
* FortiOS 7.4.1 Administration Guide: URL filter configuration