Correct physics of C14 example

[KPCCoiL]

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

The code example about radioactive isotopes does not correctly represent the physics. They decay as $2^{-t / t_{1/2}}$, not $\exp(-t \times t_{1/2})$.
This PR corrects the differential equation and the analytical solution.
Also, the vertical axis of the plot is not in percentage.

(Radioactive decay is a quantum phenomenon, by the way :) )

[ChrisRackauckas]

Thanks!