docs(ngrx/data): correct AppEntityServices example in ngrx data doc page

[wardbell]

closes #2280

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https:/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?
DOCS ONLY

[ ] 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
[ ] Other... Please describe:

What is the current behavior and new behavior?

The incorrect code example referred to NgRx Data as it used to be a long time ago. This PR brings the example up to date.

Closes #2280

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for 0faa6d9 at https://previews.ngrx.io/pr2413-0faa6d9/