chore(lts/notification_template): fix the example error

[wuzhuanhong]

What this PR does / why we need it:

Modify the way to reference environment variables in the example.

Which issue this PR fixes: (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged) fixes #xxx

Special notes for your reviewer:

Release note:

PR Checklist

• [x] Tests added/passed.

 make testacc TEST=./huaweicloud/services/acceptance/lts TESTARGS='-run TestAccNotificationTemplate_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/lts -v -run TestAccNotificationTemplate_basic -timeout 360m -parallel 4
=== RUN   TestAccNotificationTemplate_basic
=== PAUSE TestAccNotificationTemplate_basic
=== CONT  TestAccNotificationTemplate_basic
--- PASS: TestAccNotificationTemplate_basic (29.51s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/lts       29.669s

• [x] Documentation updated.
• [ ] Schema updated.

• [ ] CheckDeleted.

  • a. During query operation (Read Context) aa. Resource not found >>>>>> Paste the screenshot here <<<<<<
  • b. During delete/disassociate/unbind operation (Delete Context) ba. Resource not found >>>>>> Paste the screenshot here <<<<<<

[Lance52259]

/approve /lgtm