Fix bug in eigenvector()

The routine wrongly returned 1/lambda instead of lambda, as the leading
eigenvalue of the adjacency matrix.

src/graph/centrality/graph_eigenvector.hh

@@ -116,7 +116,7 @@ struct get_eigenvector
             }
         }
 
-        eig = 1. / norm;
+        eig = norm;
     }
 };