Dynare++ tutorial: add a note about the computation of IRFs when shocks are correlated

The treatment is different that that of Dynare. For more background, see
https://forum.dynare.org/t/difference-between-high-order-approx-in-dynare-and-dynare/16570/4

By the way, make a few minor modernizations.

[skip ci]

(cherry picked from commit 9a726e61)