edit README.md modify word from expand to extend (#2138)

Co-authored-by: guirong.ggr <guirong.ggr@dtwave-inc.com>

README.md

@@ -17,7 +17,7 @@ Dolphin Scheduler Official Website
 
 ### Design features:
 
-A distributed and easy-to-expand visual DAG workflow scheduling system. Dedicated to solving the complex dependencies in data processing, making the scheduling system `out of the box` for data processing.
+A distributed and easy-to-extend visual DAG workflow scheduling system. Dedicated to solving the complex dependencies in data processing, making the scheduling system `out of the box` for data processing.
 Its main objectives are as follows:
 
  - Associate the Tasks according to the dependencies of the tasks in a DAG graph, which can visualize the running state of task in real time.