update dataobject.md

fixex typo
Merge pull request #2882 from sanjaymundhra/patch-4

update dataobject.md
DOC: DropdownField __construct

Current docs make PHPStorm spew errors about data types.

`new CheckboxSetField('Countries'...);`

Causes `Expected The, got string`.

Simple restructure.
Merge pull request #2883 from TomSpeak/patch-1

DOC: DropdownField __construct