x/website: ref/mod's vcs-find section should mention the "mod" version control system

[mvdan]

I recently asked a question which boils down to: can I publish a Go module in plain zip archives, without using a standard VCS like git?

@bcmills helpfully pointed me towards https://golang.org/ref/mod#serving-from-proxy, which is basically what I want :)

The reason I had failed to find that section of the reference doc is because I looked in https://golang.org/ref/mod#vcs-find, which does not list "mod" in its table. It probably should, and have the mention link to the serving-from-proxy section.