Fix -Wformat compilation warning

Without the changes the gcc8 compiler complains about:

warning: format ‘%s’ expects argument of type ‘char*’, but argument 3 has type ‘std::string’ {aka ‘std::__cxx11::basic_string<char>’} [-Wformat=]
  613 |   if (fWorkspace->set(Form("CACHE_CONSTR_OF_PDF_%s_FOR_OBS_%s", fPdf->GetName(),