Fix column label for L0 write sum

Summary:
This is taken from the "Write(GB)" column in compaction stats, so the
units should be GB, not MB.

Test Plan: none

Reviewers: sdong, yhchiang, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D56889

tools/benchmark.sh

@@ -505,7 +505,7 @@ for job in ${jobs[@]}; do
     echo "Complete $job in $((end-start)) seconds" | tee -a $schedule
   fi
 
-  echo -e "ops/sec\tmb/sec\tSize-GB\tL0_MB\tSum_GB\tW-Amp\tW-MB/s\tusec/op\tp50\tp75\tp99\tp99.9\tp99.99\tUptime\tStall-time\tStall%\tTest"
+  echo -e "ops/sec\tmb/sec\tSize-GB\tL0_GB\tSum_GB\tW-Amp\tW-MB/s\tusec/op\tp50\tp75\tp99\tp99.9\tp99.99\tUptime\tStall-time\tStall%\tTest"
   tail -1 $output_dir/report.txt
 
 done