Update example usage of intervene

[Satya Rohith]

The example in readme gives a wrong impression to users that they need to set the appropriate content-type, which is not true.

import { serve, serveStatic } from "https://deno.land/x/sift@0.3.2/mod.ts";

serve({
  // You can serve a single file.
  "/": serveStatic("public/index.html", { baseUrl: import.meta.url }),
  // Or a directory of files.
  "/:filename+": serveStatic("public", { baseUrl: import.meta.url }),
  // Serve files hosted on the internet.
  // The URL to the resouce would become https://yourbucket.aws.com/profile.png
  "/profile.png": serveStatic("profile.png", {
    baseUrl: "https://yourbucket.aws.com",
  }),
  // You can modify the fetched response before returning to the request
  // by using the intervene option.
  "/style.css": serveStatic("style.css", {
    baseUrl: import.meta.url,
    // The intervene function is called with the request as first argument and
    // the fetched response as the second argument and it should return a
    // response as a result.
    intervene: (request, response) => {
      response.headers.set("content-type", "text/css; charset=utf-8");
      return response;
    },
  }),
});