[examples] fix clippy warning in dns example contract (#350)

15 jobs for master in 6 minutes and 19 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Check
passed #419480
linux-docker
check-std

00:00:29

passed #419481
linux-docker
check-wasm

00:00:26

 
  Workspace
passed #419482
linux-docker
build-std

00:00:34

passed #419483
linux-docker
build-wasm

00:00:47

passed #419485
linux-docker
clippy-std

00:00:30

passed #419486
linux-docker
clippy-wasm

00:01:45

passed #419487
linux-docker
fmt

00:00:19

passed #419484
linux-docker
test

00:03:04

 
  Examples
passed #419490
linux-docker
examples-clippy-std

00:01:02

passed #419491
linux-docker
examples-clippy-wasm

00:01:09

passed #419492
linux-docker
examples-contract-build

00:00:29

passed #419489
linux-docker
examples-fmt

00:00:14

passed #419493
linux-docker
examples-generate-metadata

00:01:08

passed #419488
linux-docker
examples-test

00:01:21

 
  Publish
passed #419494
linux-docker
publish-docs

00:01:23