[fix] Incorrect require statement in README.MD

## Describe the bug

With the move to @koa/send, the correct way to require the library is using:

const { send } = require('@koa/send')

However, the README.md documentation still refers to:
const send = require('@koa/send')

## Checklist

- [X] I have searched through GitHub issues for similar issues.
- [X] I have completely read through the README and documentation.
- [ ] I have tested my code with the latest version of Node.js and this package and confirmed it is still not working.

Start by locating the @koa/send require example in README.md and compare it with the syntax shown in the issue. Done means the README uses the correct require form for the package's current API; no test is mentioned in the issue.

Written by the indexing model from the issue text.