Trivial typo in HISTORY.md

Summary: Closes https://github.com/facebook/rocksdb/pull/2499

Differential Revision: D5324914

Pulled By: siying

fbshipit-source-id: c69827d4dddafa81e651180633943a3380cdd5bb

HISTORY.md

@@ -21,7 +21,7 @@
 * Introduce WriteOptions.low_pri. If it is true, low priority writes will be throttled if the compaction is behind.
 * `DB::IngestExternalFile()` now supports ingesting files into a database containing range deletions.
 
-### Bu Fixes
+### Bug Fixes
 * Shouldn't ignore return value of fsync() in flush.
 
 ## 5.5.0 (05/17/2017)