chore(docs): update the README.md A2A logo (a2aproject#1154)

# Description

Update the A2A logo in README.md

To preview changes go to
[Link](https://github.com/a2aproject/a2a-python/blob/da02b3c690552094e04ac3d27f4ff8e557b4561c/README.md).

- [X] Merge PR after approval

Co-authored-by: Ivan Shymko <[email protected]>