fix(example): fix an example-app's typo in app reducers - showSidenav selector

[hadrien-toma]

Fix an example-app's typo by replacing selecthowSidenav by selectShowSidenav

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

PR Type

What kind of change does this PR introduce?

It fixes a typo.

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[x] Other... Please describe: typo for an example-app's selector

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[ngrxbot]

Preview docs changes for 56046b4 at https://previews.ngrx.io/pr2414-56046b4/

[timdeschryver]

thanks @hadrien-toma !