minor fixes to perf notebook and use 2.0.0-preview
minor fixes to perf notebook and use 2.0.0-preview

kevinykuo/sparklyr,rstudio/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr