Fixed typo in benchmark.sh (#6434)

Summary:
TB =  1024 * GB
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6434

Differential Revision: D19978339

Pulled By: zhichao-cao

fbshipit-source-id: 5a89890110b23f0ebda4a95223f66da6736321ac

tools/benchmark.sh

@@ -21,7 +21,7 @@ fi
 K=1024
 M=$((1024 * K))
 G=$((1024 * M))
-T=$((1024 * T))
+T=$((1024 * G))
 
 if [ -z $DB_DIR ]; then
   echo "DB_DIR is not defined"