Add missing bibtex key Blume_Easley2006 to fix dangling citation

The resync of likelihood_ratio_process_2.md pulled in {cite}`Blume_Easley2006`
from upstream, but the shared quant-econ.bib was not synced, so the strict
build (-W) failed on "could not find bibtex key". Entry copied verbatim from
QuantEcon/lecture-python.myst.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: mmcky

lectures/_static/quant-econ.bib

@@ -2645,3 +2645,14 @@ @article{fischer2024improving
   journal={arXiv preprint arXiv:2410.16076},
   year={2024}
 }
+
+@article{Blume_Easley2006,
+  author    = {Blume, Lawrence and Easley, David},
+  title     = {If You're So Smart, Why Aren't You Rich? {B}elief Selection in Complete and Incomplete Markets},
+  journal   = {Econometrica},
+  volume    = {74},
+  number    = {4},
+  pages     = {929--966},
+  year      = {2006}
+}
+