Update formats.md

[thomoco]

Including an Avro example with DESCRIBE url in S3

Changelog category (leave one):

• Documentation (changelog entry is not required)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Including an Avro example with DESCRIBE url in S3

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

Information about CI checks: https://clickhouse.com/docs/en/development/continuous-integration/