Improve Master Format File Prefix description in PropertyKey

[calvinjia]

In PropertyKey.java, update

  public static final PropertyKey MASTER_FORMAT_FILE_PREFIX =
      new Builder(Name.MASTER_FORMAT_FILE_PREFIX)
          .setDefaultValue("_format_")
          .setDescription("The file prefix of the file generated in the journal directory "
              + "when the journal is formatted. The master will search for a file with this "
              + "prefix when determining of the journal was once formatted.")

to

  public static final PropertyKey MASTER_FORMAT_FILE_PREFIX =
      new Builder(Name.MASTER_FORMAT_FILE_PREFIX)
          .setDefaultValue("_format_")
          .setDescription("The file prefix of the file generated in the journal directory "
              + "when the journal is formatted. The master will search for a file with this "
              + "prefix when determining if the journal is formatted.")