ts: fix query async iterator definition

[borfig]

Fixes #10250

1. The async iterator is supposed to be a method and not a property;
2. According to underlying implementation, it returns an asyncIterator
   and not a query cursor.

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Summary

This PR addresses #10250

Examples

The following code now compiles:

import { Schema, model } from "mongoose";
interface Test {
  test: string;
}
const schema = new Schema<Test>({
  test: { type: String, required: true },
});
const TestModel = model<Test>("Test", schema);
async function main() {
  const query = TestModel.find({});
  for await (const test of query) {
    console.log(test);
  }
}

[vkarpov15]

Great, thanks! 👍