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?