Fixed SKIRT logo in documentation when generating on Ubuntu
Fixed SKIRT logo in documentation when generating on Ubuntu

raesjo/SKIRTcorona,raesjo/SKIRT,SKIRT/SKIRT,SKIRT/SKIRT,raesjo/SKIRTcorona,raesjo/SKIRT,SKIRT/SKIRT,raesjo/SKIRT,raesjo/SKIRTcorona