[DOCS][MINOR] Update location of Spark YARN shuffle jar

Looking at the distributions provided on spark.apache.org, I see that the Spark YARN shuffle jar is under `yarn/` and not `lib/`.

This change is so minor I'm not sure it needs a JIRA. But let me know if so and I'll create one.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #16130 from nchammas/yarn-doc-fix.

(cherry picked from commit 5a92dc76)
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>