Adding the "https://" scheme allows godoc to properly detect
the URL and provide a hyperlink for it.

Change-Id: I76dc309368c86975de01bc6e6e9196037b2114d7
Reviewed-on: https://go-review.googlesource.com/66570Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>