message: nydusd: enrich --shared-dir help message
  Also it fixes a tiny spelling mistake.
diff: @@ -210,7 +210,7 @@ fn append_fs_options(app: App<'static, 'static>) -> App<'static, 'static> {
  Arg::with_name("shared-dir")
  .long("shared-dir")
  .short("s")
  - .help("Directory shared by host and guest for passthroughfs, which also enables pathroughfs mode")
  + .help("Directory shared by host and guest for passthroughfs or a passthroughfs source for FUSE(for test and demonstration purpose), which also enables passthroughfs mode")
  .takes_value(true)
  .conflicts_with("bootstrap"),
  )
  