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

types/d3/v3/index.d.ts

@@ -3277,7 +3277,7 @@ declare namespace d3 {
             round(round: boolean): Treemap<T>;
 
             sticky(): boolean;
-            sticky(sticky: boolean): boolean;
+            sticky(sticky: boolean): Treemap<T>;
 
             mode(): string;
             mode(mode: "squarify"): Treemap<T>;