Fix typos in chapter 1

[KGHustad]

This PR fixes two minor typos in chapter 1.

Additionally, I noticed that the "complete" program listed at the end of the chapter does not work because f is not defined. It would make sense to either define f to make the program run, or to state that the program is lacking the definition of f and therefore will not run as is.

from fenics import *
mesh = RectangleMesh(Point(-1, -1), Point(1, 1), 10, 10)
V = FunctionSpace(mesh, 'P', 2)  # quadratic polynomials
bc = DirichletBC(V, 0, 'on_boundary')
u = TrialFunction(V)
v = TestFunction(V)
a = dot(grad(u), grad(v))*dx
L = f*v*dx
u = Function(V)  # unknown FEM function to be computed
solve(a == L, u, bc)
vtkfile = File('poisson.pvd'); vtkfile << u  # store solution

[matsievskiysv]

Shouldn't this be [-1,-1] \times [1,1]?

[KGHustad]

No, they're intervals.