examples: polish some README.md (#72117)
## Description
I have corrected some mistakes according to the [contribution
guide](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md).
1. Update `create-next-app` script
2. Remove redundant new line, space and double colon
3. `readme.md` → `README.md`
### Adding or Updating Examples
- [x] The "examples guidelines" are followed from our contributing doc
https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
- [x] Make sure the linting passes by running `pnpm build && pnpm lint`.
See
https://github.com/vercel/next.js/blob/canary/contributing/repository/linting.md
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Co-authored-by: Sam Ko <sam@vercel.com>
