[SPARK-3763] The example of building with sbt should be "sbt assembly" instead of "sbt compile"

In building-spark.md, there are some examples for making assembled package with maven but the example for building with sbt is only about for compiling.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2627 from sarutak/SPARK-3763 and squashes the following commits:

fadb990 [Kousuke Saruta] Modified the example to build with sbt in building-spark.md