Updated documentation

Updated documentation for step-20 Introduction section and a typo in the documentation for iterative_inverse.h.
Detailed: In the introduction of the step-20 tutorial, "Sec. Solving using the Schur complement", the IterativeInverse class is introduced and then in the following code section not used. Now the InverseMatrix is changed to IterativeInverse< .. > to match with the code of the tutorial, as used below.