docs: clarify heroes example

[reneras]

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?

Issue Number: #20904

What is the new behavior?

Instructions under Heroes HTTP subsection describes where to define the httpOptions constant.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[mary-poppins]

You can preview 8dc8db9 at https://pr21216-8dc8db9.ngbuilds.io/.

[sreevani-ship-it]

LGTM.

[mary-poppins]

You can preview 302daa1 at https://pr21216-302daa1.ngbuilds.io/.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}