Fixed wrong file name for the default Xsetup file

Commit https://github.com/sddm/sddm/commit/c449adc9b09a87ec8c4073713d6da2bba0d3dd0d introduced a wrong file name for the XSetup script, it should be Xsetup without a capitalized "s".

I fixed a similar error in the description of the SessionCommand, just for consistency.