YARN-5646. Addendum patch to include missing changes to site.xml.

(cherry picked from commit 1f344e057977cdc7765fe3af0f8c48ebed1d664e)

hadoop-project/src/site/site.xml

@@ -133,6 +133,7 @@
       <item name="Registry" href="hadoop-yarn/hadoop-yarn-site/registry/index.html"/>
       <item name="Reservation System" href="hadoop-yarn/hadoop-yarn-site/ReservationSystem.html"/>
       <item name="Graceful Decommission" href="hadoop-yarn/hadoop-yarn-site/GracefulDecommission.html"/>
+      <item name="Opportunistic Containers" href="hadoop-yarn/hadoop-yarn-site/OpportunisticContainers.html"/>
     </menu>
 
     <menu name="YARN REST APIs" inherit="top">