Update README.md

[juanludlf]

Use getButton instead of getTextField for an image field

What?

Update README.md example.

Why?

In order to properly illustrate the use of filling out PDF forms.

How?

Testing?

New Dependencies?

Screenshots

Suggested Reading?

Anything Else?

Checklist

• [x] I read CONTRIBUTING.md.
• [x] I read MAINTAINERSHIP.md#pull-requests.
• [ ] I added/updated unit tests for my changes.
• [ ] I added/updated integration tests for my changes.
• [ ] I ran the integration tests.
• [ ] I tested my changes in Node, Deno, and the browser.
• [ ] I viewed documents produced with my changes in Adobe Acrobat, Foxit Reader, Firefox, and Chrome.
• [ ] I added/updated doc comments for any new/modified public APIs.
• [x] My changes work for both new and existing PDF files.
• [ ] I ran the linter on my changes.