fix input shape of train_y

doc/getstarted/concepts/src/train.py

@@ -31,7 +31,7 @@ def event_handler(event):
 # 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]):