chore: typo of context.md (#4)

Author: atian25

docs/zh/guide/context.md

@@ -35,7 +35,7 @@ module.exports = () => {
     const start = Date.now();
     await next();
     const cost = Date.now() - start;
-    ctx.set('X-Response-Time, `${cost}ms`);
+    ctx.set('X-Response-Time', `${cost}ms`);
   }
 };
 ```
@@ -558,4 +558,4 @@ describe('test/app/extend/contex.js', () => {
 [Router]: ./router.md
 [路由]: ./router.md
 [配置]: ./config.md
-[日志]: ./logger.md
+[日志]: ./logger.md