cmd/dist: fix broken link in README

cmd/dist: fix broken link in README

While here, also update the go15bootstrap link to use the
shorter go.dev domain and https:// prefix for consistency.

For #54265.

Change-Id: I881eeda235589511a93bf47186f91f6c47c12932
Reviewed-on: https://go-review.googlesource.com/c/go/+/512720
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>