fix(example): optimistically add/remove book from collection

[AdditionAddict]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x ] 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?

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

What is the current behavior?

Closes #2417

What is the new behavior?

Example app optimistically adds/removes book from collection

Does this PR introduce a breaking change?

[ ] Yes
[ x ] No

Other information

[ngrxbot]

Preview docs changes for d91742f at https://previews.ngrx.io/pr2429-d91742f/