Broken link on migrating-existing-spas page

[M. Roche]

The link to create a single spa config on the 'migrating-existing-spas' page seems to be broken. The page containing the link is
https://single-spa.js.org/docs/migrating-existing-spas/

The link leads here:
https://single-spa.js.org/docs/migrating-existing-spas/configuration
(page not found error)

[Joel Denning]

This is fixed.