update paddlex of readme

[zhangyubo0722]

PR 类型 PR types

PR 变化内容类型 PR changes

描述 Description

提PR之前的检查 Check-list

• 这个 PR 是提交到dygraph分支或者是一个cherry-pick，否则请先提交到dygarph分支。
  This PR is pushed to the dygraph branch or cherry-picked from the dygraph branch. Otherwise, please push your changes to the dygraph branch.
• 这个PR清楚描述了功能，帮助评审能提升效率。This PR have fully described what it does such that reviewers can speedup.
• 这个PR已经经过本地测试。This PR can be convered by current tests or already test locally by you.

[paddle-bot]

Thanks for your contribution!