polish doc

doc/fluid/howto/optimization/timeline_cn.md

 # 如何使用timeline工具做性能分析
 
-1. 在训练的主循环外加上`profiler.start_profiler(...)`和`profiler.start_profiler(...)`。运行之后，代码会在`/tmp/profile`目录下生成一个profile的记录文件。
+1. 在训练的主循环外加上`profiler.start_profiler(...)`和`profiler.stop_profiler(...)`。运行之后，代码会在`/tmp/profile`目录下生成一个profile的记录文件。
 
 	**提示：**
 	请不要在timeline记录信息时运行太多次迭代，因为timeline中的记录数量和迭代次数是成正比的。

doc/fluid/howto/optimization/timeline_en.md

 # how to use timeline tool to do profile
 
-1. Add `profiler.start_profiler(...)`和`profiler.start_profiler(...)` to the main training loop. After run, the code will generate a profile record file `/tmp/profile`. **Warning**: Please do not run too many batches when use profiler to record timeline information, for the profile record will grow with the batch number.
+1. Add `profiler.start_profiler(...)`和`profiler.stop_profiler(...)` to the main training loop. After run, the code will generate a profile record file `/tmp/profile`. **Warning**: Please do not run too many batches when use profiler to record timeline information, for the profile record will grow with the batch number.
 
 	```python
     for pass_id in range(pass_num):