order issue with opt savings

[jstac]

In the optimal savings lecture https://jax.quantecon.org/opt_savings.html, the function R_sigma is used (in get_value) before it is defined.

The order should be swapped.

There might be a similar issue with the optimal investment lecture

[jstac]

Assigning @Smit-create but anyone is welcome to take this on.

[Smit-create]

Thanks John. I have noticed a couple of small other issues which I'll fix soon.