add output to fit a line demo

Author: putcn

python/paddle/v2/fluid/tests/book/test_fit_a_line.py

@@ -49,7 +49,7 @@
         avg_loss_value, = exe.run(fluid.default_main_program(),
                                   feed=feeder.feed(data),
                                   fetch_list=[avg_cost])
-
+        print(avg_loss_value)
         if avg_loss_value[0] < 10.0:
             exit(0)  # if avg cost less than 10.0, we think our code is good.
 exit(1)