[SPARK-4889] update history server example cmds

can you make this more terse
```
SPARK_HISTORY_OPTS=" -Dspark.history.fs.logDirectory=/tmp/spark-events" ./sbin/start-history-server.sh
SPARK_HISTORY_OPTS=" -Dspark.history.fs.logDirectory=hdfs:/tmp/spark-events" ./sbin/start-history-server.sh
```