style(linting): clarify the type of 2nd argument of Operator<T, R>.ca…

…ll()

- This commit is spin-off of ReactiveX#2104

src/Operator.ts

@@ -1,6 +1,7 @@
+import { Observable } from './Observable';
 import { Subscriber } from './Subscriber';
 import { TeardownLogic } from './Subscription';
 
 export interface Operator<T, R> {
- call(subscriber: Subscriber<R>, source: any): TeardownLogic;
+ call(subscriber: Subscriber<R>, source: Observable<T>): TeardownLogic;
 }