Update deeponet.md

[jstrubeAtWork]

I've had a difficult time following the example out of the box, so I

• Increased the network size and changed sigmoid -> tanh, since that worked better for me.
• Increased the number of evaluation points to be able to test the network
• added a plot of the prediction of a previously unseen function.

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.

[jstrubeAtWork]

I see the test failed, but don't understand why. Happy to make changes, but don't know which.