database/sql: add example for DB.Prepare and Tx.Prepare
database/sql: add example for DB.Prepare and Tx.Prepare

Change-Id: Ib9272a7713ed7aaf8ad54c4827be8c095763e648
Reviewed-on: https://go-review.googlesource.com/c/go/+/161677
Reviewed-by: Daniel Theophanes <14db39248b80c6688f7e9808ba5055f2e56426f3@gmail.com>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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