Update aiyagari.md: Fix reference to VFI instead of HPI

Updated the "Primitives and operators" section to correctly state that we solve the household problem using value function iteration (VFI), not Howard policy iteration (HPI). Removed the outdated reference to Ch 5 of Dynamic Programming book.

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

Author: jstac

lectures/aiyagari.md

@@ -231,7 +231,7 @@ Below we provide code to solve the household problem, taking $r$ and $w$ as fixe
 
 ### Primitives and operators
 
-We will solve the household problem using Howard policy iteration (see Ch 5 of [Dynamic Programming](https://dp.quantecon.org/)).
+We will solve the household problem using value function iteration.
 
 First we set up a `NamedTuple` to store the parameters that define a household asset accumulation problem, as well as the grids used to solve it