[DOCS] Update docker_demo.md to explicitily use Scala Version 2.11 for the demo build (#12399)

Adding -Dscala-2.11 argument to `mvn clean package` to ensure that the supported Scala Version 2.11 is used and to avoid errors like #10262

Author: derFunk

website/versioned_docs/version-0.15.0/docker_demo.md

@@ -51,7 +51,7 @@ NOTE: Make sure you've cloned the [Hudi repository](https://github.com/apache/hu
 
 ```java
 cd <HUDI_WORKSPACE>
-mvn clean package -Pintegration-tests -DskipTests
+mvn clean package -Pintegration-tests -DskipTests -Dscala-2.11
 ```
 
 ### Bringing up Demo Cluster