doc: fix OP logo on README.md

[snario]

This is the most up-to-date logo.

https://github.com/ethereum-optimism/brand-kit/blob/main/assets/svg/Profile-Logo.svg

[changeset-bot]

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[tuxcanfly]

OK. Thought I had seen some issues with raw.githubusercontent,.com but Content-type is image/svg+xml 👍🏽

[mergify]

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