Remove unnecessary global alpha variable

The α parameter is already defined with a default value (0.4) in the
create_model function, so there's no need to set it as a global variable
and pass it explicitly.

Simplified:
- model = create_model() instead of α = 0.4; model = create_model(α=α)
- model_crra = create_model() in the CRRA exercise section

Tested successfully with same results as before.

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

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

Author: jstac

lectures/cake_eating_egm_jax.md

@@ -164,9 +164,7 @@ f_prime = lambda k, α: α * k**(α - 1)
 Now we create a model instance.
 
 ```{code-cell} python3
-α = 0.4
-
-model = create_model(α=α)
+model = create_model()
 grid = model.grid
 ```
 
@@ -358,7 +356,7 @@ Now we solve for $\gamma = 1$ (log utility) and values approaching 1 from above.
 policies = {}
 endogenous_grids = {}
 
-model_crra = create_model(α=α)
+model_crra = create_model()
 
 for γ in γ_values:
     c_init = jnp.copy(model_crra.grid)