update reference style

Author: mmcky

lectures/_config.yml

@@ -37,6 +37,7 @@ latex:
 sphinx:
   extra_extensions: [sphinx_multitoc_numbering, sphinxext.rediraffe, sphinx_exercise, sphinx_togglebutton, sphinx.ext.intersphinx, sphinx_proof, sphinx_tojupyter] 
   config:
+    bibtex_reference_style: author_year
     # false-positive links
     linkcheck_ignore: ['https://doi.org/https://doi.org/10.2307/1235116', 'https://math.stackexchange.com/*', 'https://stackoverflow.com/*']
     # myst-nb config