Fixed example in reducers.md

[acb14js]

Code change related to this issue: #2233

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?

[ ] 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?

#2233

import { createAction } from '@ngrx/store';

Closes #2233

What is the new behavior?

import { createAction, props } from '@ngrx/store';

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for 94abc55 at https://previews.ngrx.io/pr2247-94abc55/