fix(timer): fix type signature

Author: luisgabriel

src/Observable.ts

@@ -147,7 +147,7 @@ export default class Observable<T> implements CoreOperators<T>  {
   static of: <T>(...values: (T | Scheduler)[]) => Observable<T>;
   static range: <T>(start: number, end: number, scheduler?: Scheduler) => Observable<number>;
   static throw: <T>(error: T) => Observable<T>;
-  static timer: (delay: number) => Observable<number>;
+  static timer: (dueTime: number, period?: number | Scheduler, scheduler?: Scheduler) => Observable<number>;
   static zip: <T>(...observables: (Observable<any> | ((...values: Array<any>) => T)) []) => Observable<T>;
   ignoreElements: () => Observable<T>;
 }

src/observables/TimerObservable.ts

@@ -5,7 +5,7 @@ import nextTick from '../schedulers/nextTick';
 
 export default class TimerObservable<T> extends Observable<T> {
 
-  static create(dueTime: number = 0, period?: number | Scheduler, scheduler?: Scheduler) {
+  static create(dueTime: number = 0, period?: number | Scheduler, scheduler?: Scheduler): Observable<number> {
     return new TimerObservable(dueTime, period, scheduler);
   }