[SPARK-14711][BUILD] Examples jar not a part of distribution.

## What changes were proposed in this pull request?

Move the spark-examples.jar from being in examples/target to examples/target/scala-2.11/jars

## How was this patch tested?

Built distribution to make sure examples jar was being included in the tarball.
Ran run-example to make sure examples were run.

Author: Mark Grover <mark@apache.org>

Closes #12476 from markgrover/spark-14711.

examples/pom.xml

@@ -297,6 +297,13 @@
           <skip>true</skip>
         </configuration>
       </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-jar-plugin</artifactId>
+        <configuration>
+          <outputDirectory>${jars.target.dir}</outputDirectory>
+        </configuration>
+      </plugin>
     </plugins>
   </build>
   <profiles>