Mention rocksdb_namespace.h in HISTORY.md (#6439)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6439

Differential Revision: D20012442

fbshipit-source-id: a7c9569826eac39cd7ea69c90f08a21dd4caa335

HISTORY.md

@@ -24,7 +24,7 @@
 ### New Features
 * Added the checksum for each SST file generated by Flush or Compaction. Added sst_file_checksum_func to Options such that user can plugin their own SST file checksum function via override the FileChecksumFunc class. If user does not set the sst_file_checksum_func, SST file checksum calculation will not be enabled. The checksum information inlcuding uint32_t checksum value and a checksum function name (string). The checksum information is stored in FileMetadata in version store and also logged to MANIFEST. A new tool is added to LDB such that user can dump out a list of file checksum information from MANIFEST (stored in an unordered_map).
 * `db_bench` now supports `value_size_distribution_type`, `value_size_min`, `value_size_max` options for generating random variable sized value. Added `blob_db_compression_type` option for BlobDB to enable blob compression.
-* Replace RocksDB namespace "rocksdb" with flag "ROCKSDB_NAMESPACE" which if is not defined, defined as "rocksdb".
+* Replace RocksDB namespace "rocksdb" with flag "ROCKSDB_NAMESPACE" which if is not defined, defined as "rocksdb" in header file rocksdb_namespace.h.
 
 ## 6.7.0 (01/21/2020)
 ### Public API Change