Improvement of Documentation in types-and-platforms/server-types/bukkit-spigot

[virgil724]

I think the documentation should include an explanation that
VERSION needs to be specified for -e BUKKIT_DOWNLOAD_URL= and -e SPIGOT_DOWNLOAD_URL=.
https://docker-minecraft-server.readthedocs.io/en/latest/types-and-platforms/server-types/bukkit-spigot/

[itzg]

Thanks for pointing that out. Will do.