Fix incorrect documentation.

[yukukotani]

Current example of id as an argument to v-drag is not working. So I added new one.

[nattyluke]

For me it still didnt work for me because there is noch event.path.
Had to update following function like this:
function checkIfIdInPath(id, path) { while( path.id !== id && path.parentNode) path = path.parentNode return path.id === id }
Also slightly changed first line in mousedown function:
if (_data.draggableElementId && !checkIfIdInPath(_data.draggableElementId, e.target)) { return }
I can upload a branch with the changes if you like (and give me permissions, or what is the correct way, forking?)