chore: update readme

[woldtwerk]

update react and react-dom version from 16 to 18

related issue: https://github.com/decaporg/gatsby-plugin-decap-cms/issues/4

Summary The readme was outdated. The correct react versions are 18

Manually initializing decap threw: i.createRoot is not a function

Test plan

Checklist

Please add a x inside each checkbox:

• [x] I have read the contribution guidelines.

A picture of a cute animal (not mandatory but encouraged)