internal/dl: change GOARCH=arm64 pretty string ARMv8 → ARM64

ARM64 makes for a better pretty string for the "arm64" architecture.
It's more clear that it's 64-bit, and it's closer to what we almost
always say.

Thanks to Cherry for making this suggestion.

For golang/go#38485.
For golang/go#36439.

Change-Id: I89c175ea2bafce6f191ae797ed982c73962ee5da
Reviewed-on: https://go-review.googlesource.com/c/website/+/344069
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

internal/dl/dl.go

@@ -362,7 +362,7 @@ var prettyStrings = map[string]string{
 	"386":    "x86",
 	"amd64":  "x86-64",
 	"armv6l": "ARMv6",
-	"arm64":  "ARMv8",
+	"arm64":  "ARM64",
 
 	"archive":   "Archive",
 	"installer": "Installer",