fix metric example (#741)

pass is the key word in python
acc.update should be in the for loop

doc/fluid/user_guides/howto/evaluation_and_debugging/evaluation/metrics.rst

@@ -22,15 +22,15 @@ Fluid中包含了常用分类指标，例如Precision, Recall, Accuracy等,更
 .. code-block:: python
 
    >>> import paddle.fluid as fluid
-   >>> labels = fluid.layers.data(name="data", shape=[1], dtype="int32")
+   >>> label = fluid.layers.data(name="label", shape=[1], dtype="int32")
    >>> data = fluid.layers.data(name="data", shape=[32, 32], dtype="int32")
    >>> pred = fluid.layers.fc(input=data, size=1000, act="tanh")
    >>> acc = fluid.metrics.Precision()
-   >>> for pass in range(PASSES):
+   >>> for pass_iter in range(PASSES):
    >>>   acc.reset()
    >>>   for data in train_reader():
-   >>>       loss, preds, labels = exe.run(fetch_list=[cost, preds, labels])
-   >>>   acc.update(preds=preds, labels=labels)
+   >>>       loss, preds, labels = exe.run(fetch_list=[cost, pred, label])
+   >>>       acc.update(preds=preds, labels=labels)
    >>>   numpy_acc = acc.eval()