doc: fix go mod index and mirror urls
Go mod index and mirror urls were missing http scheme in the design doc. As a result, it
was redirecting to
Fixes golang/go#40602
@@ -5,9 +5,9 @@ Pkg.go.dev consists of the following high-level pieces:
- A _frontend_ that presents user-facing web pages on pkg.go.dev.
- A _worker_ that populates the database with information about new modules.
- Data about new modules come from the [Go Module Index](index.golang.org),
+ Data about new modules come from the [Go Module Index](https://index.golang.org),
and the contents of these modules are downloaded from the
- [Go Module Mirror](proxy.golang.org).
+ [Go Module Mirror](https://proxy.golang.org).
- A _database_ that stores all information served on the site.
