doc/effective_go.html: fix code example
Fixes issue 6595.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/14425062
cmd/cgo: simpler fix for issue 6506.
Replaces CL 14682044.
Fixes issue 6506.

R=rsc, iant, dave
CC=golang-dev
https://codereview.appspot.com/14717043