doc: change git clone url
doc: change git clone url

The git clone url should be a Gerrit host and not Github, otherwise the
codereview command will fail.

	git-codereview: failed to load Gerrit origin: git origin must be a Gerrit host, not GitHub: https://github.com/golang/go

Change-Id: I62f62c86ee6dce0720a844fc56340135dfae8405
Reviewed-on: https://go-review.googlesource.com/117178
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

josharian/go,CAFxX/go,josharian/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,josharian/go,nawawi/go,akutz/go,nawawi/go,josharian/go,cloudflare/go,cloudflare/go,golang/go,josharian/go,akutz/go,golang/go,CAFxX/go,nawawi/go,josharian/go,josharian/go,golang/go,akutz/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,josharian/go,golang/go,nawawi/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,akutz/go,golang/go,nawawi/go