chore: add placeholder version in README.md file (#1767)

Author: ti-chi-bot

README.md

@@ -88,4 +88,4 @@ Copyright 2020 PingCAP, Inc.
 [pd]: https://github.com/pingcap/pd
 [asktug.com]: https://asktug.com/
 
-<!-- VERSION_PLACEHOLDER: v8.5.1 -->
+<!-- VERSION_PLACEHOLDER: v8.5.2 -->