Fix source vertex of BFS for Graph500-27
Fix source vertex of BFS for Graph500-27

tudelft-atlarge/graphalytics,ldbc/ldbc_graphalytics,tudelft-atlarge/graphalytics,ldbc/ldbc_graphalytics,tudelft-atlarge/graphalytics,tudelft-atlarge/graphalytics,ldbc/ldbc_graphalytics,tudelft-atlarge/graphalytics,ldbc/ldbc_graphalytics,ldbc/ldbc_graphalytics