Fix function in fit-a-line with new API (#11020)

python/paddle/fluid/tests/book/high-level-api/fit_a_line/test_fit_a_line.py

@@ -38,7 +38,7 @@ def inference_program():
     return y_predict
 
 
-def linear():
+def train_program():
     y = fluid.layers.data(name='y', shape=[1], dtype='float32')
     y_predict = inference_program()
 
@@ -104,7 +104,7 @@ def main(use_cuda):
     # Directory for saving the trained model
     params_dirname = "fit_a_line.inference.model"
 
-    train(use_cuda, linear, params_dirname)
+    train(use_cuda, train_program, params_dirname)
     infer(use_cuda, inference_program, params_dirname)