If you want to use a relative path to something in a workspace folder for the url property of a JSON schema setting item, you need to define that setting in the workspace settings (<workspace-folder-path>/.vscode/settings.json) instead of in the user settings. Related docs are here.
If you insist to define it in the user settings.json, I think you need to use an absolute path.
I think this is a sensible design. User settings apply to all workspaces, so if if relative paths were supported there, it would be an assumption that every single folder you open as a workspace would have a schema file that you'd want to use at such a path (presumably relative to the workspace folder, which I think would be a rather poor assumption, or lead to surprising users (in a bad way)).
Also note that if you want to specify schema files on a per-JSON-file-basis, you can use the $schema property in the JSON file you want to be validated to specify the schema to use.