godoc/dl: update download link from /x/build/version to /dl
godoc/dl: update download link from /x/build/version to /dl

Change-Id: Id2baaa87ab23e27ce5018271c8bb4e73750fb437
Reviewed-on: https://go-review.googlesource.com/125137
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools