[DOCS-3726] Replace EventSourceResponse with EventSource

[jrodewig]

Description

Replaces the EventSourceResponse class with EventSource in the README. TheEventSourceResponse class was removed with #184.

Motivation and context

Keep the docs up-to-date.

How was the change tested?

N/A

Screenshots (if appropriate):

Change types

• • Bug fix (non-breaking change that fixes an issue)
• • New feature (non-breaking change that adds functionality)
• • Breaking change (backwards-incompatible fix or feature)

Checklist:

• • My code follows the code style of this project.
• • My change requires a change to Fauna documentation.
• • My change requires a change to the README, and I have updated it accordingly.

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