This commit performs replace double quote to backquote,
so now all examples looks consistent.

Change-Id: I8cf760ce1bdeff9619a88e531161b9516385241b
GitHub-Last-Rev: e3e636cebbf41528b8a73f9a3fe5afa10876f964
GitHub-Pull-Request: golang/go#28879
Reviewed-on: https://go-review.googlesource.com/c/150397Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>