Change image source in select_op documentation

Updated the image source for the select_op workflow.

Author: Echo-Nie

docs/design/concurrent/select_op.md

@@ -254,7 +254,7 @@ only one case will be executed.
 ### select_op flow
 
 <p align="center">
-<img src="https://raw.githubusercontent.com/PaddlePaddle/Paddle/develop/doc/fluid/images/select_op_workflow.png"/><br/>
+<img src="https://raw.githubusercontent.com/PaddlePaddle/docs/blob/develop/docs/design/concurrent/images/select_op_workflow.png"/><br/>
 </p>
 
 The select algorithm is inspired by golang's select routine.  Please refer to