Update stateful-plugins-with-widget-factory.md

The part about retrieving the plugin instance from the dom element using .data() was vague about the key used: it said "the plugin name" without mentioning that the full name is needed, including the namespace and a hyphen as a separator. The code example was plain wrong, using only the bare plugin name, which would not work.

Please feel free to improve the phrasing of this part:
> (namespace plus a hyphen plus the plugin name)