Update link for kvstore subsystem

[aj-stein-nist]

The current URL in the README file points to that code in a no-longer existent branch, migration. This now points it to main.

[thomas-fossati]

hey AJ, good to see you here, thanks for the patch!

Could you please do the DCO thing, i.e.:

In your local fork, run:

git rebase HEAD~1 --signoff

and then push again:

git push --force-with-lease origin patch-1

(If it's too much trouble, I'll force the DCO check to pass anyway.)

[aj-stein-nist]

I guess I will have to edit locally and not in the web browser now. 😆