Fix link to NonFinalGenericMethods docs in the Existentials doc

[jerryjrchen]

The original version was broken in two ways:

1. On docs.swift.org, the link points to non-existent https://docs.swift.org/embedded/documentation/embedded/existentials/NonFinalGenericMethods.md. You can see this for yourself by visiting https://docs.swift.org/embedded/documentation/embedded/existentials and finding this link:

2. In a locally generated DocC archive, the link points to non-existent https://developer.apple.com/documentation/embeddedswift/NonFinalGenericMethods, rather than the page in the same documentation archive