removed given fonts to graphviz, using default fonts on the system

doc/intro/introduction.rst

@@ -141,9 +141,9 @@ objects in the platform. In the example above, you can identify some examples.
 .. graphviz:: 
 
     digraph hierarchy {
-      graph [fontname="helvetica", compound=true, splines=polyline]
-      node [fontname="helvetica", shape=record, style=filled, fillcolor=gray95]
-      edge [fontname="helvetica"]
+      graph [compound=true, splines=polyline]
+      node [shape=record, style=filled, fillcolor=gray95]
+      edge []
 
       subgraph "algorithm_cluster" {
         1[label = "{Dataformat|...|+user\n+name\n+version}"]