fix: createSelectorFactory should return a MemoizedSelector

[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?

[ ] 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 #1849

What is the new behavior?

Does this PR introduce a breaking change?

[x] Yes
[ ] No

BREAKING CHANGE:
The return type of the createSelectorFactory function is now a MemoizedSelector instead of a Selector

[ngrxbot]

Preview docs changes for cb50067 at https://previews.ngrx.io/pr1883-cb50067/

[ngrxbot]

Preview docs changes for 360a2e1 at https://previews.ngrx.io/pr1883-360a2e1/