[BUGFIX beta] Added link to deprecations page for missing forward slash deprecation

Amendment to #9388

Wait for https://github.com/emberjs/website/pull/1760 to go live so we can be sure the URL I link to is correct.
Just deployed the website, looks like http://emberjs.com/guides/deprecations/#toc_deprecate-location-hash-paths-that-don-t-include-a-forward-slash-e-g-foo-not-foo is the winning path...