fix(schematics): correct spec description in reducer template

[koumatsumoto]

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)
• Docs have 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
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #1265

What is the new behavior?

Correct description in template according to the proposal.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Closes #1265

[coveralls]

Coverage remained the same at 88.615% when pulling 880d13e on kouMatsumoto:correct-description-in-reducer-spec-template into 4223ff9 on ngrx:master.