[DOCS] Update docs to not suggest to package Spark before running tests.

## What changes were proposed in this pull request?

Update docs to not suggest to package Spark before running tests.

## How was this patch tested?

Not creating a JIRA since this pretty small. We haven't had the need to run mvn package before mvn test since 1.6 at least, or so I am told. So, updating the docs to not be misguiding.

Author: Mark Grover <mark@apache.org>

Closes #15572 from markgrover/doc_update.