Update HDFS Federation setup document, for incorrect property name for secondary name node http address

HDFS Federation setup documentation is having incorrect property name for secondary namenode http port

It is mentioned as

<property>
    <name>dfs.namenode.secondaryhttp-address.ns1</name>
    <value>snn-host1:http-port</value>
  </property>
  <property>
    <name>dfs.namenode.rpc-address.ns2</name>
    <value>nn-host2:rpc-port</value>
  </property>

Actual property should be dfs.namenode.secondary.http-address.ns.

Because of this documentation error, when the document is followed and user tries to setup HDFS federated cluster, secondary namenode will not be started and also
hdfs getconf -secondarynamenodes throw's an exception