Fix broken reference in aiyagari.md: Replace opt_savings_2 with Dynamic Programming book link

Replace the broken cross-reference to opt_savings_2 (which doesn't exist in this PR) with a direct link to the Dynamic Programming book at dp.quantecon.org where Howard policy iteration is discussed in detail.

This fixes the build warning:
aiyagari.md:689: WARNING: unknown document: 'opt_savings_2'

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jstac

lectures/aiyagari.md

@@ -686,7 +686,7 @@ plt.show()
 
 In this lecture, we used value function iteration to solve the household problem.
 
-An alternative is Howard policy iteration (HPI), which is discussed in detail in {doc}`opt_savings_2`.
+An alternative is Howard policy iteration (HPI), which is discussed in detail in [Dynamic Programming](https://dp.quantecon.org/).
 
 HPI can be faster than VFI for some problems because it uses fewer but more computationally intensive iterations.