docs: Gantt type should not be a string

[watery]

From the Data > Overview section of the Storybook;

Here is a schematic code for configuring an activity chart:

 var gantt = new Gantt( { type: 'Gantt.type.ACTIVITY_CHART', ... });

But if type is expressed as a string, the component results in "No data available in table", while removing the quotes makes it work.