update HISTORY.md for DeleteRange bug fix

Summary:
fixed in #2799
Closes https://github.com/facebook/rocksdb/pull/2805

Differential Revision: D5734324

Pulled By: ajkr

fbshipit-source-id: a285d4e84bf1018dc2257fd6c3e7c075a7243263

HISTORY.md

@@ -16,6 +16,7 @@
 * Fix wrong latencies in `rocksdb.db.get.micros`, `rocksdb.db.write.micros`, and `rocksdb.sst.read.micros`.
 * Fix incorrect dropping of deletions during intra-L0 compaction.
 * Fix transient reappearance of keys covered by range deletions when memtable prefix bloom filter is enabled.
+* Fix potentially wrong file smallest key when range deletions separated by snapshot are written together.
 
 ## 5.7.0 (07/13/2017)
 ### Public API Change