all_predecessors(): fix docstring

This fixes issue #619

src/graph_tool/topology/__init__.py

@@ -2156,7 +2156,7 @@ def all_predecessors(g, dist_map, pred_map, weights=None, epsilon=1e-8):
     dist_map : :class:`~graph_tool.VertexPropertyMap`
         Vertex property map with the distances from ``source`` to all other
         vertices.
-    pred_map : :class:`~graph_tool.VertexPropertyMap` (optional, default: None)
+    pred_map : :class:`~graph_tool.VertexPropertyMap`
         Vertex property map with the predecessors in the search tree.
     weights : :class:`~graph_tool.EdgePropertyMap` (optional, default: None)
         The edge weights.