Correct MySQL backend version in Breeze docs

[simonprydden]

Correct the version number in example code blocks

root@d20033cda4df:/opt/airflow# breeze --python 3.8 --backend mysql --mysql-version 8

Provided outdated value 8 for parameter mysql_version. Will use 8.0 instead Good version of Docker: 24.0.6.
Good version of docker-compose: 2.23.0

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