Fix slack link in the issue template

[jeremi]

Why:

This PR fixes: issue link

What does your app/demo do?:

Just fix the slack link when reporting issues

How Has This Been Tested?

I tested the link and it worked

Have you included a README.md file in your example?

N/A

[weaviate-git-bot]

To avoid any confusion in the future about your contribution to Weaviate, we work with a Contributor License Agreement. If you agree, you can simply add a comment to this PR that you agree with the CLA so that we can merge.

beep boop - the Weaviate bot 👋🤖

PS:
Are you already a member of the Weaviate Slack channel?

[jeremi]

I agree.