[SPARK-20796] the location of start-master.sh in spark-standalone.md is wrong

[https://issues.apache.org/jira/browse/SPARK-20796](https://issues.apache.org/jira/browse/SPARK-20796)


the location of start-master.sh in spark-standalone.md should be "sbin/start-master.sh" rather than "bin/start-master.sh".

Author: liuzhaokun <liu.zhaokun@zte.com.cn>

Closes #18027 from liu-zhaokun/sbin.

(cherry picked from commit 99452df4)
Signed-off-by: Sean Owen <sowen@cloudera.com>