design/25530-notary: fix GoCenter vs Artifactory
design/25530-notary: fix GoCenter vs Artifactory

JFrog Artifactory is the (among other things) Go module proxy
that someone might run on their own local network.
GoCenter is the publicly-available instance JFrog runs.

Change-Id: I9fa030d7571fac50eb4d84b729bd82a516e4a798
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/167537
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/proposal,golang/proposal