west blobs: fix subcmd help

Without this patch, "west blobs -h" says:

  {list,fetch,clean} Select the sub-command to execute. Sub-commands
  available: - list: list binary blobs - fetch: fetch and store binary
  blobs - clean: remove fetched binary blobs

With this patch:

  {list,fetch,clean}    sub-command to execute

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>