docs: Updated mysql secret generation command

[mkaramuk]

By the change of mysql helm chart version the provided secret must contain at least three keys:

• mysql-root-password
• mysql-replication-password
• mysql-password

Otherwise mysql chart won't install. More information: https://artifacthub.io/packages/helm/bitnami/mysql#mysql-common-parameters

Checklist

• [ ] The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• [ ] Links to related issues (if applicable)
• [ ] Tests for the changes have been added/updated (if applicable)
• [ ] Docs related to the changes have been added/updated (if applicable)