update 5.5 change log
update 5.5 change log

Summary:
update bug fixed.
Closes https://github.com/facebook/rocksdb/pull/2434

Differential Revision: D5218601

Pulled By: lightmark

fbshipit-source-id: 1f86b2c93345673612381081537d464e7d12e434

vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb