Docs: Improve `using dep ...` section

[izzyreal]

Currently https://scala-cli.virtuslab.org/docs/reference/directives/#dependency looks like this:

Some using dep examples have org::name syntax, while others have org:name. Though I now realise that using org:name is actually valid, it's confusing at best that these 2 syntaxes are mixed with no explanation.

Moreover, since https://scala-cli.virtuslab.org/docs/reference/directives is the reference documentation, I would expect it to be more detailed about the using dep specifications.