Update automatic build secrets using ephemeral machine Dockerfile secret casing

[Coddo-Python]

Summary of changes

Changed the casing of the property read by the jq command in the example given on using ephemeral machines to automatically inject build secrets from Name to name

The current example doesn't work since the output of the flyctl secrets list --json command outputs name for every environment variable, not Name, causing the jq command to simply return null.

Fixes #2215