Mention PR 8206 in HISTORY.md (#8210)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/8210

Reviewed By: akankshamahajan15

Differential Revision: D27887612

Pulled By: ltamasi

fbshipit-source-id: 0db8d0b6047334dc47fe30a98804449043454386

HISTORY.md

@@ -2,6 +2,7 @@
 ## Unreleased
 ### Bug Fixes
 * Fixed a bug in handling file rename error in distributed/network file systems when the server succeeds but client returns error. The bug can cause CURRENT file to point to non-existing MANIFEST file, thus DB cannot be opened.
+* Fixed a data race between insertion into memtables and the retrieval of the DB properties `rocksdb.cur-size-active-mem-table`, `rocksdb.cur-size-all-mem-tables`, and `rocksdb.size-all-mem-tables`.
 
 ## 6.20.0 (04/16/2021)
 ### Behavior Changes