Fix next SWIP ID (apache#12217)

Author: wu-sheng

docs/en/swip/readme.md

@@ -68,7 +68,7 @@ All accepted and proposed SWIPs can be found in [here](https://github.com/apache
 
 ## Known SWIPs
 
-Next SWIP Number: 8
+Next SWIP Number: 9
 
 ### Accepted SWIPs