docs(operators): Add a simple example in doc string (#4636)

* docs(operators): describe your change...

Add much simpler example to help understand what `switchMap` can do. It is easy to understand for document reader.

* update the sample to following TS grammar

* updated according to suggestion

1. mark sample code in typescript
2. space between of(1, 2, 3)
3. outputs

* docs(operators): updated the sample code in docs

src/internal/operators/switchMap.ts

@@ -36,6 +36,22 @@ export function switchMap<T, R, O extends ObservableInput<any>>(project: (value:
  * subsequent inner Observables.
  *
  * ## Example
+ * Generate new Observable according to source Observable values
+ * ```typescript
+ * import { of } from 'rxjs';
+ * import { switchMap } from 'rxjs/operators';
+ *
+ * const switched = of(1, 2, 3).pipe(switchMap((x: number) => of(x, x ** 2, x ** 3)));
+ * switched.subscribe(x => console.log(x));
+ * // outputs
+ * // 1
+ * // 1
+ * // 2
+ * // 4
+ * // 8
+ * // ... and so on
+ * ```
+ *
  * Rerun an interval Observable on every click event
  * ```javascript
  * import { fromEvent, interval } from 'rxjs';