Fix hadoop.version in building-spark.md

Couple of mvn build examples use `-Dhadoop.version=VERSION` instead of actual version number

Author: Alexander Pivovarov <apivovarov@gmail.com>

Closes apache#15440 from apivovarov/patch-1.

docs/building-spark.md

@@ -91,13 +91,13 @@ Examples:
     ./build/mvn -Pyarn -Phadoop-2.3 -Dhadoop.version=2.3.0 -DskipTests clean package
 
     # Apache Hadoop 2.4.X or 2.5.X
-    ./build/mvn -Pyarn -Phadoop-2.4 -Dhadoop.version=VERSION -DskipTests clean package
+    ./build/mvn -Pyarn -Phadoop-2.4 -Dhadoop.version=2.4.0 -DskipTests clean package
 
     # Apache Hadoop 2.6.X
     ./build/mvn -Pyarn -Phadoop-2.6 -Dhadoop.version=2.6.0 -DskipTests clean package
 
     # Apache Hadoop 2.7.X and later
-    ./build/mvn -Pyarn -Phadoop-2.7 -Dhadoop.version=VERSION -DskipTests clean package
+    ./build/mvn -Pyarn -Phadoop-2.7 -Dhadoop.version=2.7.0 -DskipTests clean package
 
     # Different versions of HDFS and YARN.
     ./build/mvn -Pyarn -Phadoop-2.3 -Dhadoop.version=2.3.0 -Dyarn.version=2.2.0 -DskipTests clean package