modify comments about flush_queue_

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/4850

Differential Revision: D13591940

Pulled By: sagar0

fbshipit-source-id: 617794e0a41d0f4554d40871180b061e84189fc5

db/db_impl.h

@@ -1364,7 +1364,7 @@ class DBImpl : public DB {
   // compacted. Consumers of these queues are flush and compaction threads. When
   // column family is put on this queue, we increase unscheduled_flushes_ and
   // unscheduled_compactions_. When these variables are bigger than zero, that
-  // means we need to schedule background threads for compaction and thread.
+  // means we need to schedule background threads for flush and compaction.
   // Once the background threads are scheduled, we decrease unscheduled_flushes_
   // and unscheduled_compactions_. That way we keep track of number of
   // compaction and flush threads we need to schedule. This scheduling is done