changed slack link to code contributions repo as requested

[flashrod]

Before submitting this pull request, check the changes to see it's only the changes you made intentionally If there are changes to other lines you didn't make deliberately, it's possible that your IDE made the changes with a utility like prettier. Next time, make sure that you only add your changes by using git add -p and rather than git add Contributors.md

If you're doing something in the checklist below, put an x inside [ ] so that - [ ] becomes - [x]

• [x] I had fun going through this tutorial (ノ^o^)ノ and learned on the way ٩(＾◡＾)۶
• [x] There are some things I'd like to improve in this tutorial. I have written them below.
• [ ] There were steps where I had errors while following this tutorial. I have written them below.