[fix] Incorrect require statement in README.MD

[exortech]

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

• I have searched through GitHub issues for similar issues.
• 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.