DOC : fixed minor error in using topological sort

`topological_sort` is a function in `nx` not an instance method
of a directed graph.

docs/source/parallel/dag_dependencies.rst

@@ -117,7 +117,7 b' on which it depends:'
     
     In [6]: results = {}
     
-    In [7]: for node in G.topological_sort():
+    In [7]: for node in nx.topological_sort(G):
        ...:    # get list of AsyncResult objects from nodes
        ...:    # leading into this one as dependencies
        ...:    deps = [ results[n] for n in G.predecessors(node) ]