docs: add clarifying example to job dispatch command docs

https://developer.hashicorp.com/nomad/docs/commands/job/dispatch

The docs here don't make it clear that `nomad job dispatch ` is referring to JobID (as opposed to the job filename). It's also not clear that the job must be "run" first before "dispatch". Let's fixup the docs so these points are clear.

Open the linked Nomad job dispatch command documentation and review the usage section and examples. Update the wording so it identifies the argument as the JobID rather than a job filename and states that the job must be run before dispatching. Done means both prerequisites are clear to readers.

Written by the indexing model from the issue text.