[SPARK-22866] [K8S] Fix path issue in Kubernetes dockerfile

The path was recently changed in apache#19946, but the dockerfile was not updated.

Author: foxish

resource-managers/kubernetes/docker/src/main/dockerfiles/spark-base/Dockerfile

@@ -38,7 +38,7 @@ COPY jars /opt/spark/jars
 COPY bin /opt/spark/bin
 COPY sbin /opt/spark/sbin
 COPY conf /opt/spark/conf
-COPY dockerfiles/spark-base/entrypoint.sh /opt/
+COPY kubernetes/dockerfiles/spark-base/entrypoint.sh /opt/
 
 ENV SPARK_HOME /opt/spark