CheckboxItem: checkbox-trashcan icon Does Not Display in the Sampler (bu...

...t do in the sample). Enyo-DCO-1.1-Signed-off-by: Brooke Peterson <brooke.peterson@lge.com>

###Issue:
In Sampler - CheckboxItem, customized image src 'checkbox-trashcan' icon did not display.

###Fix: 
In moonstone/samples/CheckboxItemSample.js:
Change the src relative path to absolute path.