docs(widgets-object): fix example to use real widget instead of nonexistent "date" (DCMS-477)

[sempostma]

Closes laikacms/decap-cms#500

The Object widget's own example config used widget: "date", which doesn't exist in the widget registry (only datetime does, via decap-cms-widget-datetime). Fixed the example to use widget: "datetime" with date_format: "MM/DD/YYYY" (the actual date-only option name; format sets the combined datetime storage format).

Grepped content/docs/ for other stray widget: "date" examples — this was the only occurrence; other date-named fields already correctly use widget: "datetime".