Fix xent call in mnist tutorial code

Fixes #857.

tutorials/image/mnist/convolutional.py

@@ -228,7 +228,7 @@ def main(_):
   # Training computation: logits + cross-entropy loss.
   logits = model(train_data_node, True)
   loss = tf.reduce_mean(tf.nn.sparse_softmax_cross_entropy_with_logits(
-      logits, train_labels_node))
+      labels=train_labels_node, logits=logits))
 
   # L2 regularization for the fully connected parameters.
   regularizers = (tf.nn.l2_loss(fc1_weights) + tf.nn.l2_loss(fc1_biases) +