[SPARK-20455][DOCS] Fix Broken Docker IT Docs

## What changes were proposed in this pull request?

Just added the Maven `test`goal.

## How was this patch tested?

No test needed, just a trivial documentation fix.

Author: Armin Braun <[email protected]>

Closes #17756 from original-brownbear/SPARK-20455.

Author: original-brownbear

docs/building-spark.md

@@ -232,7 +232,7 @@ Once installed, the `docker` service needs to be started, if not already running
 On Linux, this can be done by `sudo service docker start`.
 
     ./build/mvn install -DskipTests
-    ./build/mvn -Pdocker-integration-tests -pl :spark-docker-integration-tests_2.11
+    ./build/mvn test -Pdocker-integration-tests -pl :spark-docker-integration-tests_2.11
 
 or