Apps Spec Broken Link

[danaelhe]

Apps documentation points to a broken link for App specification documentation

For documentation on app specifications (AppSpec objects), please refer 
to the [product documentation](https://www.digitalocean.com/docs/app-platform/references/app-specification-reference/).

[andrewsomething]

Looks like that should be https://docs.digitalocean.com/products/app-platform/reference/app-spec/

[danaelhe]

thank you! will do a pr now