[SPARK-19769][DOCS] Update quickstart instructions

## What changes were proposed in this pull request?

This change addresses the renaming of the `simple.sbt` build file to
`build.sbt`. Newer versions of the sbt tool are not finding the older
named file and are looking for the `build.sbt`. The quickstart
instructions for self-contained applications is updated with this
change.

## How was this patch tested?

As this is a relatively minor change of a few words, the markdown was checked for syntax and spelling. Site was built with `SKIP_API=1 jekyll serve` for testing purposes.

Author: Michael McCune <msm@redhat.com>

Closes #17101 from elmiko/spark-19769.

(cherry picked from commit bf5987cb)
Signed-off-by: Sean Owen <sowen@cloudera.com>