Clarify forkjoin error behavior

[tongfa]

I read the learn-rxjs documentation on forkJoin, then read the main documentation at:
https://rxjs.dev/api/index/function/forkJoin

which states:
"""
If any given observable errors at some point, forkJoin will error as well and immediately unsubscribe from the other observables.
"""

I felt that the learn-rxjs documentation did not properly reflect that statement. After reading the learn-rxjs documentation I was under the impression all the inner observables would have completed successfully, including any side effects (i.e. server side state change) they may have caused. But that's not true.

This PR is my suggestion on how to improve the learn-rxjs documentation to properly reflect what the main documentation states.

I also fixed a couple example links that were in the same area.