Make the PDESystem tutorial materialization simpler, without `reshape`

[asinghvi17]

Checklist

• [ ] Appropriate tests were added
• [ ] Any code changes were done in a way that does not break public API
• [ ] All documentation related to code changes were updated
• [ ] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [ ] Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[asinghvi17]

I guess it's better to broadcast, but this at least indicates how you do it while respecting dimensionality.

[sathvikbhagavan]

Thanks! Can you do this in other places in the documentation?