docs(testing.md): fix type MiddleWare not found to MiddleWare url

[khk37601]

The existing import statement causes Not Found.

change
import type { Middleware } from "https://deno.land/x/mod.ts"; to
import { testing, Middleware } from "https://deno.land/x/oak/mod.ts";

[kitsonk]

The documentation is correct. You should not import type only imports in Deno with import and instead use import type.

Can you be more specific on what error you are encountering?