add old version into TOC

Author: HumphreyYang

lectures/_toc.yml

@@ -67,6 +67,7 @@ parts:
   - file: dyn_stack
   - file: calvo
   - file: calvo_gradient
+  - file: calvo_gradient_old  
   - file: opt_tax_recur
   - file: amss
   - file: amss2

lectures/calvo_gradient_old.md

@@ -11,7 +11,7 @@ kernelspec:
   name: python3
 ---
 
-# A Model of Calvo
+# A Model of Calvo (Old)
 
 This lecture describes a  linear-quadratic versions of a model that Guillermo Calvo {cite}`Calvo1978` used to illustrate the **time inconsistency** of optimal government
 plans.