Fix typo in the comment of log_empty_ (#8711)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/8711

Reviewed By: riversand963

Differential Revision: D30566761

Pulled By: jay-zhuang

fbshipit-source-id: dd4690f5e2af2d263ed75ea1b9ed24692fe81362

db/db_impl/db_impl.h

@@ -2015,7 +2015,7 @@ class DBImpl : public DB {
   // accessed from the same write_thread_ without any locks. With
   // two_write_queues writes, where it can be updated in different threads,
   // read and writes are protected by log_write_mutex_ instead. This is to avoid
-  // expesnive mutex_ lock during WAL write, which update log_empty_.
+  // expensive mutex_ lock during WAL write, which update log_empty_.
   bool log_empty_;
 
   ColumnFamilyHandleImpl* persist_stats_cf_handle_;