Fix typo in GraphWidget's docstring

This fixes issue #275.

src/graph_tool/draw/gtk_draw.py

@@ -150,7 +150,7 @@ class GraphWidget(Gtk.DrawingArea):
             given via the ``vertex_<prop-name>`` parameters, where ``<prop-name>`` is
             the name of the property.
         eprops : dict (optional, default: ``None``)
-            Dictionary with the vertex properties. Individual properties may also be
+            Dictionary with the edge properties. Individual properties may also be
             given via the ``edge_<prop-name>`` parameters, where ``<prop-name>`` is
             the name of the property.
         vorder : :class:`~graph_tool.PropertyMap` (optional, default: ``None``)