Update README: change `cleanup` -> `clear`

[bantic]

What:

Typo fix on the README.

The code example on the README uses a function cleanup, but I think it is supposed to be clear, as cleanup is not part of the RenderResult type but clear is.

Why:

Clarity

How:

Find/replace on README.md

Checklist:

• [x] Tests n/a
• [x] TypeScript definitions updated n/a
• [x] Ready to be merged