Fixed slack channel link

[diogofcunha]

Thanks for contributing to react-virtualized!

Here is a short checklist of additional things to keep in mind before submitting:

• Please make sure your pull request description makes it very clear what you're trying to accomplish. If it's a bug fix, please also provide a failing test case (if possible). In either case, please add additional unit test coverage for your changes. :)
• Run tests locally (npm test) to ensure that your change did not break linting or functionality.
• Be sure you have notifications setup so that you'll see my code review responses. (I may ask you to make some adjustments before merging.)