fix: update type of BunAdapter json body

[rayz1065]

This code currently shows a typing error due to incompatible requests:

import { Bot, webhookCallback } from "grammy";

const bot = new Bot("");
Bun.serve({
  routes: {
    "/webhook": {
      POST: webhookCallback(bot, "bun"),
    },
  },
});

This pull request aligns the type of json() with bun.

[KnorpelSenf]

LGTM