fix(customPropTypes): itemsShorthand -> collectionShorthand

[levithomason]

This prop type checker was renamed in all components, but the customPropType method itself somehow remained with the old name. Fixes these issues:

Where:

    /** Shorthand array of props for Menu. */
    items: customPropTypes.collectionShorthand,

[levithomason]

/cc @jcarbo can you double check me here, am I renaming this in the right direction or should the components have been updated? From the latest PR comments at #618 I think this is correct.

[jeffcarbs]

You're right. Not sure how I missed that.

[levithomason]

No worries, you and I both ;)