internal/dl: use go.dev domain in documentation
internal/dl: use go.dev domain in documentation

Since golang.org/dl/ always redirects to go.dev/dl/,
update the documentation accordingly.

Updates golang/go#34864.

Change-Id: Ied245b8765a2570609ea30441ffe7676b04796aa
Reviewed-on: https://go-review.googlesource.com/c/website/+/401094
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Auto-Submit: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website