HDDS-1165. Document generation in maven should be configured on execution level. Contributed by Anu Engineer.

hadoop-hdds/docs/pom.xml

@@ -31,6 +31,7 @@ http://maven.apache.org/xsd/maven-4.0.0.xsd">
   <dependencies>
 
   </dependencies>
+
   <build>
     <plugins>
       <plugin>
@@ -43,8 +44,6 @@ http://maven.apache.org/xsd/maven-4.0.0.xsd">
               <goal>exec</goal>
             </goals>
             <phase>compile</phase>
-          </execution>
-        </executions>
             <configuration>
               <executable>${shell-executable}</executable>
               <workingDirectory>${project.build.directory}</workingDirectory>
@@ -54,6 +53,8 @@ http://maven.apache.org/xsd/maven-4.0.0.xsd">
                 <argument>${project.build.directory}</argument>
               </arguments>
             </configuration>
+          </execution>
+        </executions>
       </plugin>
       <plugin>
         <groupId>org.apache.rat</groupId>