Update HISTORY.md for PR 7888 (#7890)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/7890

Reviewed By: ajkr

Differential Revision: D26005509

Pulled By: ltamasi

fbshipit-source-id: e7eb732180d447900788d0e3a17dfd1c3f1e708a

HISTORY.md

@@ -1,6 +1,5 @@
 # Rocksdb Change Log
 ## Unreleased
-* Fix a race condition between DB startups and shutdowns in managing the periodic background worker threads. One effect of this race condition could be the process being terminated.
 
 ## 6.17.0 (01/15/2021)
 ### Behavior Changes
@@ -9,6 +8,7 @@
 
 ### Bug Fixes
 * Version older than 6.15 cannot decode VersionEdits `WalAddition` and `WalDeletion`, fixed this by changing the encoded format of them to be ignorable by older versions.
+* Fix a race condition between DB startups and shutdowns in managing the periodic background worker threads. One effect of this race condition could be the process being terminated.
 
 ### Public API Change
 * Add a public API WriteBufferManager::dummy_entries_in_cache_usage() which reports the size of dummy entries stored in cache (passed to WriteBufferManager). Dummy entries are used to account for DataBlocks.