samll equation issue in 'nitsch.ipynb & .py'

[Yanjun96]

Hi, In the Chapter 1, about Weak imposition of Dirichlet conditions for the Poisson problem. The equation (9) and (10) now is:

The D should change to below:

I do not know how to push my changes directly. Maybe I need to find help of git tutorial.

Regards yanjun