README.md: link to Update Documentation broken

[christophberger]

The link "Upgrade Documentation" in README.md, section "For Seafile 7.x.x" points to a non-existing page.

I found the "6.3 upgrade to 7.0" document at: https://download.seafile.com/published/seafile-manual/docker/6.3%20upgrade%20to%207.0.md

Current link is: https://download.seafile.com/published/support/docker/6.3%20upgrade%20to%207.0.md

[renfeipeng]

Thank you for your reminder. I will update the README.