Fix WriteShockDecomp2Excel.m

See https://forum.dynare.org/t/bug-in-write-xls-option-of-shock-decomposition-after-estimation/11097 On MAC, we rely on xlwrite from https://fr.mathworks.com/matlabcentral/fileexchange/38591-xlwrite--generate-xls-x--files-without-excel-on-mac-linux-win without checking whether Excel is installed. If we want to keep this, we need to clearly spell out that users need to install that file. @rattoma What do you think?