Rename HUDPanel.cs to HudPanel.cs (#216)

The class name was changed from "HUDPanel" to "HudPanel" so the script filename also needs to change - Unity fails to load or run the script as it is currently written.

The alternative to renaming the file would be to rename the class back to HUDPanel, but I can see it's at least used in the interface, and I don't know where else. Changing the file name is the easiest/least painful way to fix this.

Co-authored-by: LaurieCheers-unity <73140792+LaurieCheers-unity@users.noreply.github.com>