Update readme in light of affjax driver changes

[JordanMartinez]

Description of the change

Resolves https://github.com/purescript-contrib/purescript-affjax/pull/171#issuecomment-1105259382 Since this is another part of #171, I don't think this needs a changelog entry.

CC @thomashoneyman

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Checklist:

• [ ] Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• [ ] Linked any existing issues or proposals that this pull request should close
• [ ] Updated or added relevant documentation in the README and/or documentation directory
• [ ] Added a test for the contribution (if applicable)

[JordanMartinez]

:ping_pong: @thomashoneyman Looks like this hasn't been approved yet.