Update app.md and listeners.md

[ProgramRipper]

Added some missing words.
补充了一些漏字、漏词。

---

BTW, there is a strange phenomenon on "headers" page. Both en and zh
顺便一提，在“标头(Headers)”这一页面上有一奇怪的现象，在英语和中文页面中都有。


The contents of the red and blue boxes are exactly the same, but I didn't find out the reason from the source code. I think this may be due to the rendering error of the Markdown file.
红蓝两框的内容是完全一致的，但我没有在源码中找到原因。我想这也许是因为Markdown文件的渲染出错。

[ahopkins]

Thanks for the PR. I think I have noticed this once before about the headers page. I will see if I can figure it out.

[ZinkLu]

@ProgramRipper Thanks for the PR. 感谢老铁。

[ahopkins]

I figured it out.... it seems to be a problem when going from a list to columns:

- `content-length`
- `content-type`
- `connection`
- `transfer-encoding`


---:1

Any other header that you would like to set can be done either in the route handler, or a response middleware.

:--:1

...
:---

If I add an arbitrary line of text after the list, it renders as expected 🤷‍♂️

[ahopkins]