use always project-path, instead of project-dir

usage tells you

    Usage: swagger --project-path PATH [--output-path PATH]...
    
      -p, --project-path     Path containing the php files with Swagger annotations

if you go with `--project-path` you'll get an error which is telling you, that `--project-dir must be provided`. since we have everything ending on `path` i changed all the occurrences of `project-dir` into `project-path`