docs(aio): updates doc error in guide/http

[bennetthardwick]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

https://angular.io/guide/http#testing-for-errors

Currently the documentation says to call req.error when forcing a network failure, but instead gives an example using req.flush.

Issue Number: #23453

What is the new behavior?

The documentation now shows two correct examples. One using req.error, simulating a network failure, and one using req.flush, simulating a 404.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[bennetthardwick]

I signed it!

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CLAs look good, thanks!

[IgorMinar]

thanks!

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