Fix failing CI

[Smit-create]

Extension error (quantecon_book_theme):
Handler <function add_to_context at 0x7fb4973f1ee0> for event 'html-page-context' threw an exception (exception: 'pdf_book_name')
Traceback (most recent call last):
  File "/usr/share/miniconda3/envs/quantecon/lib/python3.9/site-packages/sphinx/events.py", line 94, in emit
    results.append(listener.handler(self.app, *args))
  File "/usr/share/miniconda3/envs/quantecon/lib/python3.9/site-packages/quantecon_book_theme/__init__.py", line 188, in add_to_context
    context["pdf_book_path"] = "/_pdf/" + context["pdf_book_name"] + ".pdf"
KeyError: 'pdf_book_name'

More details: https://github.com/QuantEcon/lecture-python-intro/actions/runs/4042400225/jobs/6950055949

[mmcky]

Thanks @Smit-create I will check this out