speech: fix docs to use correct event

[evancohen]

After a little bit of head scratching I'm pretty sure that the docs are incorrect. The requestStream emits a 'response' event: index.js#L484

Right now they the docs claim it is 'data'. The event name should either be changed or the docs should be updated to reflect the actual name to prevent further confusion.

Note: this may impact #1631

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[googlebot]

CLAs look good, thanks!

[evancohen]

Nope. Please disregard.