docs: use static zip function as an Observable creator. (#26790)

The existing example makes it seem like zip is a pipeable operator. It can be used this way, but I think that is for backwards compatibility. You can achieve the same functionality by using it as an Observable creator. I think this also makes the example clearer.

PR Close #26790

aio/content/examples/practical-observable-usage/src/backoff.ts

@@ -5,10 +5,9 @@ import { retryWhen, map, mergeMap } from 'rxjs/operators';
 
 function backoff(maxTries, ms) {
  return pipe(
-   retryWhen(attempts => range(1, maxTries)
+   retryWhen(attempts => zip(range(1, maxTries), attempts)
      .pipe(
-       zip(attempts, (i) => i),
+       map(([i]) => i * i),
-       map(i => i * i),
        mergeMap(i =>  timer(i * ms))
      )
    )