Update option example for Svelte 4 (#496)

Author: zqianem

content/tutorial/02-advanced-svelte/09-special-elements/09-svelte-options/README.md

@@ -26,6 +26,6 @@ The options that can be set here are:
 - `accessors={true}` — adds getters and setters for the component's props
 - `accessors={false}` — the default
 - `namespace="..."` — the namespace where this component will be used, most commonly `"svg"`
-- `tag="..."` — the name to use when compiling this component as a custom element
+- `customElement="..."` — the name to use when compiling this component as a custom element
 
 Consult the [API reference](https://svelte.dev/docs) for more information on these options.