Fix type for placeholder in select component

[thomasthiebaud]

First of all, thank you for your contribution! :-)

Please makes sure that these checkboxes are checked before submitting your PR, thank you!

• Make sure that you propose PR to right branch: bugfix for master, feature for latest active branch feature-x.x.
• Make sure that you follow antd's code convention.
• Run npm run lint and fix those errors before submitting in order to keep consistent code style.
• Rebase before creating a PR to keep commit history clear.
• Add some descriptions and refer relative issues for you PR.

In the documentation, the placeholder can be string | ReactNode but not in the code. This PR update the type definition to include string and ReactNode

[ant-design-bot]

Deploy preview for ant-design ready!

Built with commit a4d0209

https://deploy-preview-10282--ant-design.netlify.com