toc help is incorrect

[jasmainak]

Proof attached:

$ jupyter-book toc --help
usage: jupyter-book [-h] [--quiet] path_book

Execute all of the notebooks in a specified folder.

positional arguments:
  path_book   The path to a Jupyter Book.

optional arguments:
  -h, --help  show this help message and exit
  --quiet     Whether to suppress the final message box.

@choldgraf let me know if it's okay for you this way. I'm raising issues as I notice them.

[choldgraf]

this is perfect - the CLI is brand new and there are bound to be some bugs in there :-)