Move xxhash64 checksum support to 'Unreleased' section (#4627)

Summary:
Move the line `Add xxhash64 checksum support` to `Unreleased` section because it has not been released to 5.17.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4627

Differential Revision: D12944123

Pulled By: riversand963

fbshipit-source-id: 2762857065b9e741c64ff8b6116ca62fe31891d8

HISTORY.md

@@ -7,6 +7,7 @@
 * Introduced DB option `atomic_flush`. If true, RocksDB supports flushing multiple column families and atomically committing the result to MANIFEST. Useful when WAL is disabled.
 * Added `num_deletions` and `num_merge_operands` members to `TableProperties`.
 * Added "rocksdb.min-obsolete-sst-number-to-keep" DB property that reports the lower bound on SST file numbers that are being kept from deletion, even if the SSTs are obsolete.
+* Add xxhash64 checksum support
 
 ### Bug Fixes
 * Fix corner case where a write group leader blocked due to write stall blocks other writers in queue with WriteOptions::no_slowdown set.
@@ -17,11 +18,6 @@
 * Fix the bug where user comparator was sometimes fed with InternalKey instead of the user key. The bug manifests when during GenerateBottommostFiles.
 * Fix a bug in WritePrepared txns where if the number of old snapshots goes beyond the snapshot cache size (128 default) the rest will not be checked when evicting a commit entry from the commit cache.
 
-## 5.17.1 (10/30/2018)
-## Unreleased
-### New Features
-* Add xxhash64 checksum support
-
 ## 5.17.0 (10/05/2018)
 ### Public API Change
 * `OnTableFileCreated` will now be called for empty files generated during compaction. In that case, `TableFileCreationInfo::file_path` will be "(nil)" and `TableFileCreationInfo::file_size` will be zero.