Fix comment doesn't describe behavior

Comment refers to a person that knows bob and code would return a name, but this would only work if the triple
`person foaf:name bob .`
is part of the dataset. As this is a very uncommon way to model a `foaf:knows` the code was adjusted to match the description.

docs/intro_to_graphs.rst

@@ -105,7 +105,7 @@ node, not a generator:
     # get any name of bob
     name = g.value(bob, FOAF.name)
     # get the one person that knows bob and raise an exception if more are found
-    mbox = g.value(predicate = FOAF.name, object=bob, any=False)
+    person = g.value(predicate = FOAF.knows, object=bob, any=False)
 
 
 :class:`~rdflib.graph.Graph` methods for accessing triples