docs(config): update preview.https type (#17586)

Author: Jainex17

docs/config/preview-options.md

@@ -46,7 +46,7 @@ Set to `true` to exit if port is already in use, instead of automatically trying
 
 ## preview.https
 
-- **Type:** `boolean | https.ServerOptions`
+- **Type:** `https.ServerOptions`
 - **Default:** [`server.https`](./server-options#server-https)
 
 Enable TLS + HTTP/2. Note this downgrades to TLS only when the [`server.proxy` option](./server-options#server-proxy) is also used.