correct the demo code for dense_vector label input

doc/getstarted/concepts/use_concepts_cn.rst

@@ -111,7 +111,7 @@ PaddlePaddle支持不同类型的输入数据，主要包括四种类型，和
     # define training dataset reader
     def train_reader():
         train_x = np.array([[1, 1], [1, 2], [3, 4], [5, 2]])
-        train_y = np.array([-2, -3, -7, -7])
+        train_y = np.array([[-2], [-3], [-7], [-7]])
         def reader():
             for i in xrange(train_y.shape[0]):
                 yield train_x[i], train_y[i]