update history.md for fixing the bug that skips keys

Summary: Closes #1986

Differential Revision: D4699152

Pulled By: siying

fbshipit-source-id: b18c32c

HISTORY.md

@@ -10,6 +10,7 @@
 * Memtable flush can be avoided during checkpoint creation if total log file size is smaller than a threshold specified by the user.
 
 ### Bug Fixes
+* Fix the bug that iterator may skip keys
 * Remove calling fallocate with FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE flag to circumvent a kernel bug that changes file size after this call on XFS
 
 ## 5.2.0 (02/08/2017)