Fix minor typo in comment

Summary:
mean -> meant
Closes https://github.com/facebook/rocksdb/pull/3202

Differential Revision: D6426443

Pulled By: sagar0

fbshipit-source-id: adaf07218580ee6903986fa5686de92f43f420e1

include/rocksdb/listener.h

@@ -214,7 +214,7 @@ struct ExternalFileIngestionInfo {
 };
 
 // A call-back function to RocksDB which will be called when the compaction
-// iterator is compacting values. It is mean to be returned from
+// iterator is compacting values. It is meant to be returned from
 // EventListner::GetCompactionEventListner() at the beginning of compaction
 // job.
 class CompactionEventListener {