docs(fix): header sample code (QwikDev#3360)

Author: himorishige

packages/docs/src/routes/qwikcity/endpoints/index.mdx

@@ -49,7 +49,7 @@ export const onGet: RequestHandler = async ({ status }) => {
 
 ```tsx
 export const onPost: RequestHandler = async ({ headers }) => {
-  headers.set('X-My-Custom-Header': 'Hello World');
+  headers.set('X-My-Custom-Header', 'Hello World');
 };
 ```