README: planed -> planned

[dmitrii-badretdinov]

Pull request summary

Fixed typo: planed -> planned
The Table of Contents doesn't seem to work, so I updated the link too. Nevermind, ToC works.

Pull request checklist

• Have you followed the guidelines in our Contributing document? (It doesn't exist)
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? (A typo fix is hard to test, so there are no tests.)
• Have you successfully tested your changes locally?