[RF] Fix plotting of nested RooSimultaneous with projection data

Just as with the generation of datasets from a RooSimultaneous, the
right way to iterate over the index category components is not to call
`servers()`, but to use `RooSimultaneous::flattenedCatList()`.

Closes root-project#12652.

roofit/roofitcore/src/RooSimultaneous.cxx

@@ -696,7 +696,7 @@ RooPlot* RooSimultaneous::plotOn(RooPlot *frame, RooLinkedList& cmdList) const
     // Determine if any servers of the index category are in the projectedVars
     RooArgSet projIdxServers ;
     bool anyServers(false) ;
-    for (const auto server : _indexCat->servers()) {
+    for (const auto server : flattenedCatList()) {
       if (projectedVars.find(server->GetName())) {
         anyServers=true ;
         projIdxServers.add(*server) ;