Fix missing equation label 'bell2mcmc' in mccall_fitted_vfi.md

Added the label 🏷️ bell2mcmc to the Bellman operator equation and
converted it from $$ delimiters to the {math} directive to support
cross-references. This fixes the build warning:

  WARNING: equation not found: bell2mcmc

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

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

Author: jstac

lectures/mccall_fitted_vfi.md

@@ -96,7 +96,9 @@ The model is the same as in the {doc}`discrete case <mccall_model_with_sep_marko
 
 In the {doc}`discrete case <mccall_model_with_sep_markov>`, we ended up iterating on the Bellman operator
 
-$$
+```{math}
+:label: bell2mcmc
+
     (Tv_u)(w) =
     \max
     \left\{
@@ -106,7 +108,7 @@ $$
         \right),
         u(c) + \beta(Pv_u)(w)
     \right\}
-$$
+```
 
 where