Slack URL updated

Previous URL was 404

.template-helpers/get-help.md

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-[the Docker Community Forums](https://forums.docker.com/), [the Docker Community Slack](https://blog.docker.com/2016/11/introducing-docker-community-directory-docker-community-slack/), or [Stack Overflow](https://stackoverflow.com/search?tab=newest&q=docker)
+[the Docker Community Forums](https://forums.docker.com/), [the Docker Community Slack](http://dockr.ly/slack), or [Stack Overflow](https://stackoverflow.com/search?tab=newest&q=docker)