Typos on demo website

[Miraboreasu]

https://fenicsproject.org/olddocs/dolfin/latest/python/demos/nonlinear-poisson/demo_nonlinear-poisson.py.html

It will be great if someone can fill with the nonlinear Poisson PDE.

[dokken]

Fix pushed in: Bitbucket
Should be:
- \nabla\cdot((1 + u^2) \nabla u) = f \quad {\rm in}\, \Omega,
u = 1 \quad {\rm on}\, \Gamma_D,
\nabla u\cdot n = 0 \quad {\rm on}\, \Gamma_N.