[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>

docs/running-on-yarn.md

@@ -518,7 +518,7 @@ instructions:
 pre-packaged distribution.
 1. Locate the `spark-<version>-yarn-shuffle.jar`. This should be under
 `$SPARK_HOME/common/network-yarn/target/scala-<version>` if you are building Spark yourself, and under
-`lib` if you are using a distribution.
+`yarn` if you are using a distribution.
 1. Add this jar to the classpath of all `NodeManager`s in your cluster.
 1. In the `yarn-site.xml` on each node, add `spark_shuffle` to `yarn.nodemanager.aux-services`,
 then set `yarn.nodemanager.aux-services.spark_shuffle.class` to