HistoryServer documentation refers to non-existent spark-history-server.sh script

The examples for how to start a history server refer to a spark-history-server.sh script that doesn't exist; afaict it never did.

I believe the examples mean to refer to ./sbin/start-history-server.sh, and should be updated to reflect that the log directory should be specified via -Dspark.history.fs.logDirectory in $SPARK_HISTORY_OPTS, not via a command-line argument.