docs(marketing): Fix duplicated link

[dtaylor84]

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
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

The link for "You might not need NgRx by Mike Ryan at AngularConnect 2018" is a duplicate of the link to the "NgRx at ng-conf 2018" playlist.

What is the new behavior?

Fix link to point at the correct video

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[ngrxbot]

Preview docs changes for da47c18 at https://previews.ngrx.io/pr2399-da47c18/

[timdeschryver]

Thanks @dtaylor84 !