fix Streamjs GroupingResult

GroupingResult return an array of element
Exemple : 
Stream.of({'firstname': 'john', 'lastname':'doe'}).groupBy(obj => obj.lastname)['doe']
=> is an Array

types/streamjs/index.d.ts

@@ -126,7 +126,7 @@ declare namespace Stream {
 	}
 
 	export interface GroupingResult<T> {
-		[index: string]: T
+		[index: string]: T[]
 	}
 
 	export interface Iterator<T> {