Update HISTORY for SeekForPrev bug fix (facebook#5925)

Summary:
Update history for the bug fix in facebook#5907
Pull Request resolved: facebook#5925

Differential Revision: D17952605

Pulled By: maysamyabandeh

fbshipit-source-id: 609afcbb2e4087f9153822c4d11193a75a7b0e7a

Author: Maysam Yabandeh

HISTORY.md

@@ -3,6 +3,7 @@
 ### Bug Fixes
 * Revert the feature "Merging iterator to avoid child iterator reseek for some cases (#5286)" since it might cause strange results when reseek happens with a different iterator upper bound.
 * Fix a bug in BlockBasedTableIterator that might return incorrect results when reseek happens with a different iterator upper bound.
+* Fix a bug when partitioned filters and prefix search are used in conjunction, ::SeekForPrev could return invalid for an existing prefix. ::SeekForPrev might be called by the user, or internally on ::Prev, or within ::Seek if the return value involves Delete or a Merge operand.
 
 ## 6.5.0 (9/13/2019)
 ### Bug Fixes