docs: Fix help command line parameter

[barbeau]

Summary:

With this change it now matches https://github.com/MobilityData/gtfs-realtime-validator/tree/master/gtfs-realtime-validator-lib#run-it-yourself (i.e., it should be in camelCase).

Expected behavior:

Command line parameters in the help output should match the actual command line parameters.

Please make sure these boxes are checked before submitting your pull request - thanks!

• [x] Run the unit tests with mvn test to make sure you didn't break anything
• [ ] Format the title like "Fix # - " (for example - "Fix #1111 - Check for null value before using field")
• [ ] Linked all relevant issues
• [ ] Include screenshot(s) showing how this pull request works and fixes the issue(s)

[isabelle-dr]

Just opened #95 for the same purpose 😅

Thank you Sean, merging here