Fix bug in Graph.load() with file-like objects

This fixes a small typo bug, which prevented loading from file-like
objects.

src/graph_tool/core.py

@@ -804,7 +804,7 @@ class Graph(object):
         if isinstance(file_name, str):
             props = self.__graph.ReadFromFile(file_name, None, file_format)
         else:
-            props = self.__graph.ReadFromFile("", file_name, format)
+            props = self.__graph.ReadFromFile("", file_name, file_format)
         for name, prop in props[0].iteritems():
             self.vertex_properties[name] = PropertyMap(prop, self, "v")
         for name, prop in props[1].iteritems():