src/sage/graphs/base/boost_graph.pyx: Add missing doctest prompts

Author: Matthias Koeppe

src/sage/graphs/base/boost_graph.pyx

@@ -561,15 +561,15 @@ cpdef bandwidth_heuristics(g, algorithm='cuthill_mckee'):
 
     Given a wrong algorithm::
 
-        from sage.graphs.base.boost_graph import bandwidth_heuristics
+        sage: from sage.graphs.base.boost_graph import bandwidth_heuristics
         sage: bandwidth_heuristics(graphs.PathGraph(3), algorithm='tip top')
         Traceback (most recent call last):
         ...
         ValueError: unknown algorithm 'tip top'
 
     Given a graph with no edges::
 
-        from sage.graphs.base.boost_graph import bandwidth_heuristics
+        sage: from sage.graphs.base.boost_graph import bandwidth_heuristics
         sage: bandwidth_heuristics(Graph())
         (0, [])
         sage: bandwidth_heuristics(graphs.RandomGNM(10,0))                              # needs networkx