fix(schematics): add schematics to devDependencies

[timdeschryver]

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?

[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?

Currently, when you run ng add @ngrx/schematics the package will be added to dependencies.

What is the new behavior?

This change, adds the @ngrx/schematics package to the devDependencies.

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

Schematics docs

[ngrxbot]

Preview docs changes for 3130665 at https://previews.ngrx.io/pr2784-31306659/