Fix mis-spelling (#8001)

Summary:
concurrnet -> concurrent

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8001

Reviewed By: ajkr

Differential Revision: D26659381

Pulled By: riversand963

fbshipit-source-id: 890d102d1cf836ed3b183da66d3d56a3158017d0

include/rocksdb/memtablerep.h

@@ -120,7 +120,7 @@ class MemTableRep {
     return true;
   }
 
-  // Same as ::InsertWithHint, but allow concurrnet write
+  // Same as ::InsertWithHint, but allow concurrent write
   //
   // If hint points to nullptr, a new hint will be allocated on heap, otherwise
   // the hint will be updated to reflect the last insert location. The hint is