Fixed typo on Value mismatch error in db_test (#6587)

Summary:
The debug is supposed to print out two keys to show the value mismatch, which was compared just a few lines above.

However, the actual print-out is the same values (so they obviously won't be mismatched)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/6587

Reviewed By: riversand963

Differential Revision: D23025279

Pulled By: ajkr

fbshipit-source-id: 4c6c35bc60b273f13c08b5464b6f690d8a5cfe41

db/db_test.cc

@@ -3146,7 +3146,7 @@ static bool CompareIterators(int step, DB* model, DB* db,
       fprintf(stderr, "step %d: Value mismatch for key '%s': '%s' vs. '%s'\n",
               step, EscapeString(miter->key()).c_str(),
               EscapeString(miter->value()).c_str(),
-              EscapeString(miter->value()).c_str());
+              EscapeString(dbiter->value()).c_str());
       ok = false;
     }
   }