Change book_path to path_to_book

Jupyter-Book looks to have changed the argument: https://jupyterbook.org/customize/config.html?highlight=issue#add-a-button-to-suggest-edits

Should fix PSLmodels#2456

docs/_config.yml

@@ -5,7 +5,7 @@ logo: PSL.svg
 exclude_patterns: [guide/make, guide/templates, recipes/md_src, '**/README.md']
 repository:
   url: https://github.com/PSLmodels/Tax-Calculator
-  book_path: docs
+  path_to_book: docs
   branch: master
 html:
   use_edit_page_button: true
@@ -18,4 +18,4 @@ sphinx:
   extra_extensions          : ['sphinx.ext.autodoc', 'sphinx.ext.mathjax',
                                'sphinx.ext.viewcode', 'sphinx.ext.napoleon',
                                'alabaster']   # A list of extra extensions to load by Sphinx.
-  config                    :   # key-value pairs to directly over-ride the Sphinx configuration
+  config                    :   # key-value pairs to directly over-ride the Sphinx configuration