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 <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: dmitshur

src/cmd/dist/README

@@ -16,6 +16,6 @@ Because of backward compatibility, although the steps above say Go 1.20.6,
 in practice any release ≥ Go 1.20.6 but < Go 1.x will work as the bootstrap base.
 Releases ≥ Go 1.x are very likely to work as well.
 
-See golang.org/s/go15bootstrap for more details about the original bootstrap
-and https://go.dev/cl/54265 for details about later bootstrap version bumps.
+See https://go.dev/s/go15bootstrap for more details about the original bootstrap
+and https://go.dev/issue/54265 for details about later bootstrap version bumps.