[examples][doc] Fix sign of pressure in Stokes example

While the exact sign is not important mathematically, the example used the physically wrong sign. Thus this is also uncommon in literature. The physically correct form of the Stokes equation is -\Delta u + \grad p = f which, after partial integration leads to a -div(v)p term in the weak form.

This issues exists since the example was added. Maybe the initial version of the example was implemented based on the book by Braess. There, indeed the uncommon sign is used for the Stokes as well as for the Navier-Stokes equation.

Not surprisingly, the uncommon convention leads to confusing results. E.g. for the lid driven cavity flow, the pressure decreases towards the corner where the fluid is driven to. Similarly, for Poiseuille flow in a pipe the fluid is driven towards the high pressure side.