corrected the data-hide syntax for dynamic checkboxes

[tombradford]

https://osc.github.io/ood-documentation/latest/how-tos/app-development/interactive/dynamic-form-widgets.html#hiding-entire-elements

changes when-unchecked to when-un-checked as suggested on OSC/ondemand#3199

(whitespace changes as a result of a local vim pluign to remove trailing whitespaces - i can remove these changes if required)

tested successfully using ondemand 3.1.7:

---

form:
  - checkbox_test
  - path

attributes:
  path:
    widget: path_selector
  checkbox_test:
    widget: check_box
    html_options:
      data:
        hide-path-when-un-checked: true

[johrstrom]

Thanks!