Minor fix to plotting script.

src/plot_train.py

@@ -146,7 +146,7 @@ def main():
     parser.add_argument('--labels', nargs='+', default=[],
         help='Labels for each model used in the plot. There should be as many '
              'labels as models.')
-    parser.add_argument('--metric', default='perplexity-diff',
+    parser.add_argument('--metric', default='cross-entropy-diff',
         help='Which metric to plot on the y-axis. Default is difference in '
              'cross entropy between the model and the lower bound on the '
              'validation set.')