docs: graph: remove unnecessary `graph_update()' call

The sample code calls `get_revision()' followed by `graph_update()',
but the documentation and source code indicate that `get_revision()'
already calls `graph_update()' for you.

Signed-off-by: Michael Witten <mfwitten@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-history-graph.txt

@@ -115,7 +115,6 @@ struct commit *commit;
 struct git_graph *graph = graph_init(opts);
 
 while ((commit = get_revision(opts)) != NULL) {
-	graph_update(graph, commit);
 	while (!graph_is_commit_finished(graph))
 	{
 		struct strbuf sb;