migrate_example : BeerTerm plugin has the wrong description for the fields

The description of fields for BeerTerm plugin have been copied from the BeerUser.php.
Should be changed to something more suitable to these fields.

$fields = [
      'style' => $this->t('Account ID'),
      'details' => $this->t('Blocked/Allowed'),
      'style_parent' => $this->t('Registered date'),
      // These values are not currently migrated - it's OK to skip fields you
      // don't need.
      'region' => $this->t('Region the style is associated with'),
      'hoppiness' => $this->t('Hoppiness of the style'),
    ];