docs(entity): fix selectedUserId type

[markostanimirovic]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/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?

[ ] 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?

In @ngrx/entity examples, interface User has id property which is of type string, but selectedUserId has type number or number | null instead of string | null.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[ngrxbot]

Preview docs changes for 417ddef at https://previews.ngrx.io/pr3017-417ddefc/