Update maven-site-plugin used by benchmark module to 3.8.2

The benchmark module currently has no parent, adjust the version used
here to the one used by all the other jgit Maven modules.

Change-Id: I8807a694fe23f8f131d1d22a58a3e18874d756cc
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.packaging/pom.xml

         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-site-plugin</artifactId>
-          <version>3.7.1</version>
+          <version>3.8.2</version>
         </plugin>
       </plugins>
     </pluginManagement>