Update index.d.ts
Update index.d.ts

According to the official documentation

> Like other classes in D3, layouts follow the method chaining pattern where setter methods return the layout itself, allowing multiple setters to be invoked in a concise statement.

And d3.layout.treemap.sticky specifically https://github.com/d3/d3-3.x-api-reference/blob/master/Treemap-Layout.md#sticky  should return Treemap<T>.

This file should comply with the actual API, why it is not?

Submiting this MR to your consideration
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