fix(doc): add more TS types

Author: dynst

apps/rxjs.dev/content/guide/subject.md

@@ -118,7 +118,7 @@ To achieve that with explicit calls to `connect()`, we write the following code:
 ```ts
 import { interval, Subject, multicast } from 'rxjs';
 
-const source = interval(500);
+const source: Observable<number> = interval(500);
 const subject = new Subject<number>();
 const multicasted = source.pipe(multicast(subject));
 let subscription1, subscription2, subscriptionConnect;