fix nce docstring for python 3, test=develop, test=document_fix (#2225)

fix nce docstring for python 3

doc/fluid/api_cn/layers_cn/nce_cn.rst

@@ -43,15 +43,15 @@ nce
 
     window_size = 5
     words = []
-    for i in xrange(window_size):
-        words.append(fluid.layers.data(
-            name='word_{0}'.format(i), shape=[1], dtype='int64'))
+    for i in range(window_size):
+        words.append(fluid.data(
+            name='word_{0}'.format(i), shape=[-1, 1], dtype='int64'))
 
     dict_size = 10000
     label_word = int(window_size / 2) + 1
 
     embs = []
-    for i in xrange(window_size):
+    for i in range(window_size):
         if i == label_word:
             continue
 
@@ -64,7 +64,7 @@ nce
             num_total_classes=dict_size, param_attr='nce.w_0',
             bias_attr='nce.b_0')
 
-    # 或使用自定义分布
+    #or use custom distribution
     dist = np.array([0.05,0.5,0.1,0.3,0.05])
     loss = fluid.layers.nce(input=embs, label=words[label_word],
             num_total_classes=5, param_attr='nce.w_1',