docs(project): update mvvm.zh-CN.md

[xudihui]

This document has been very helpful to me. After reading it repeatedly, I realized that simply changing it would make it easier for users to understand

Before submitting a pull request, please make sure the following is done...

• [x] Ensure the pull request title and commit message follow the Commit Specific in English.
• [x] Fork the repo and create your branch from master or formily_next.
• [x] If you've added code that should be tested, add tests!
• [x] If you've changed APIs, update the documentation.
• [x] Ensure the test suite passes (npm test).
• [x] Make sure your code lints (npm run lint) - we've done our best to make sure these rules match our internal linting guidelines.

Please do not delete the above content

---

What have you changed?