Improve Worker Data Folder property key description

[calvinjia]

In PropertyKey.java

Update

public static final PropertyKey WORKER_DATA_HOSTNAME =
      new Builder(Name.WORKER_DATA_HOSTNAME)
          .setDescription("The hostname of Alluxio worker data service").build();

to

public static final PropertyKey WORKER_DATA_HOSTNAME =
      new Builder(Name.WORKER_DATA_HOSTNAME)
          .setDescription("The hostname of Alluxio worker data service.").build();