godoc/dl: update some pretty GOARCH strings

Fixes golang/go#19082

Change-Id: Id0e985eeff543ad24ff969ff8c94e086a7bc9303
Reviewed-on: https://go-review.googlesource.com/46916
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
1 file changed