Update link to point to numba.html instead of parallelization.html

The parallel programming lecture reorganization moved the Numba
parallelization exercise to numba.md. Update the link accordingly.

Related to QuantEcon/lecture-python-programming.myst#429

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

Author: jstac

lectures/jax_intro.md

@@ -674,7 +674,7 @@ jnp.allclose(z_vmap, z_mesh)
 :label: jax_intro_ex2
 ```
 
-In the Exercise section of [a lecture on Numba and parallelization](https://python-programming.quantecon.org/parallelization.html), we used Monte Carlo to price a European call option.
+In the Exercise section of [a lecture on Numba](https://python-programming.quantecon.org/numba.html), we used Monte Carlo to price a European call option.
 
 The code was accelerated by Numba-based multithreading.