Update handler.md

Need a closing parentheses to close the JSON.stringify 
```
app.get('/', () => new Response(
    JSON.stringify({
        'vtuber': [
            'Shirakami Fubuki',
            'Inugami Korone'
        ]
    }, {
        headers: {
            'Content-Type': 'application/json'
        }
    })
)
```

Author: mafi020

docs/concept/handler.md

@@ -102,7 +102,7 @@ app.get('/', () => new Response(
             'Shirakami Fubuki',
             'Inugami Korone'
         ]
-    }, {
+    }), {
         headers: {
             'Content-Type': 'application/json'
         }