[SPARK-21382] The note about Scala 2.10 in building-spark.md is wrong.

[https://issues.apache.org/jira/browse/SPARK-21382](https://issues.apache.org/jira/browse/SPARK-21382)
There should be "Note that support for Scala 2.10 is deprecated as of Spark 2.1.0 and may be removed in Spark 2.3.0",right?

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

Closes #18606 from liu-zhaokun/new07120923.