Fix broken post urls

[lewisnyman]

I noticed that at some point my blog post link structure changed from:
/blog/year-month-date-slug to /blog/slug

For example:
/blog/2014-01-03-hello-jekyll is now /blog/hello-jekyll

I'm trying to use Netlify redirects to fix this, but the placeholder syntax described on this docs page just doesn't want to work.

[lewisnyman]

Here's the example above in the deploy preview. It redirects to /blog/:slug