chore: fix playground HMR (#12572)

Author: Rich-Harris

playgrounds/demo/ssr-dev.js

@@ -22,7 +22,7 @@ polka()
 	.use(async (req, res) => {
 		const template = fs.readFileSync(path.resolve(__dirname, 'index.html'), 'utf-8');
 		const transformed_template = await vite.transformIndexHtml(req.url, template);
-		const { default: App } = await vite.ssrLoadModule('./src/main.svelte');
+		const { default: App } = await vite.ssrLoadModule('/src/main.svelte');
 		const { head, body } = render(App);
 
 		const html = transformed_template