[RF] put name of digraphs from graphVizTree in double quotes, fix #7067.
[RF] put name of digraphs from graphVizTree in double quotes, fix #7067.

When there are spaces in the titles of RooFit objects, RooFit generated
incorrect graphViz code. By enclosing all titles in quotes, the graphWiz
code can be successfully used to produce images as follows, even with
whitespaces in the names of the RooFit objects:

dot -Tpng tree_example.dot -o example1.png

root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root