Fix the spelling of Nicolson

[bangerth]

As pointed out correctly by @olegrog in #15078, the proper spelling of Phyllis Nicolson is not "Nicholson". There are several more occurrences in the library worth fixing:

include/deal.II/algorithms/theta_timestepping.h:   * schemes, the Crank-Nicholson scheme and linear combinations of those. The
include/deal.II/algorithms/theta_timestepping.h:   * <li> #theta=½: Crank-Nicholson scheme
include/deal.II/algorithms/theta_timestepping.h:   * For fixed #theta, the Crank-Nicholson scheme is the only second order
examples/step-31/doc/intro.dox:evaluation of the residual is nothing else than a Crank-Nicholson scheme,

[olegrog]

It seems that I have already included all such fixes in #15078.

[marcfehling]

Closed by #15083.