readme: remove note about koa@3 as tests work

Author: jonathanong

README.md

@@ -16,8 +16,6 @@
 [downloads-image]: http://img.shields.io/npm/dm/koa-send.svg?style=flat-square
 [downloads-url]: https://npmjs.org/package/koa-send
 
-> NOTE: not yet tested to be compatible with Koa v3.
-
 Koa static file serving middleware.
 
 > Notice: We recommend using `@koa/send` for installation, as both `koa-send` and `@koa/send` refer to the same module. However, in our next major versions bumps, we will deprecate `koa-send` and only maintain the module under `@koa/send`.