clarify the `--registry-url` and `--project-dir` description

e.g. 'current registry' and the project dir has two defaults.

```
--registry-prompt-name string: Prompt name in the registry (required)
--registry-prompt-version string: Version of the prompt to pull from the registry (optional; defaults to latest)
--registry-url string: Registry URL (default: current registry)
--project-dir string: Project directory (default: current directory) (default ".")
```