Update docs/DOWNLOAD_SNAPSHOT_JAR.md with the ZIP file/JAR extraction step

[maximearmstrong]

Is your feature request related to a problem? Please describe. @barbeau noticed that we are missing a step to extract the JAR file from the ZIP archive produced by GitHub in the equivalent document for the GTFS Realtime Validator. After verification, the artifact generated by GitHub in the test package document workflow of this project is also a ZIP archive for this repo, not a JAR file. we should update the documentation with an additional step.

Proposed solution Update the documentation with an additional step to extract the JAR file from the ZIP archive.