Fix bad flag ref in dns add NS

[kobajagi]

Description

Fixes #759

Checklist

(For exoscale contributors)

• Changelog updated (under Unreleased block, and add the Pull Request #number for each bit you add to the CHANGELOG.md)
• Testing

Testing

> go run . dns add NS external-dns-test.jns.vc -n tmp -s ns1.exoscale.ch
 ✔ Adding DNS record "NS" to "external-dns-test.jns.vc"... 0s
Record "NS" was created successfully to "external-dns-test.jns.vc"

[imiric]

Ah, pflag strikes again... 😅

@kobajagi Is there a reason why the dns commands aren't using the cliCommand abstraction as other commands? It seems that it would give us type safety that would avoid these string lookup issues.

[kobajagi]

Ah, pflag strikes again... 😅

@kobajagi Is there a reason why the dns commands aren't using the cliCommand abstraction as other commands? It seems that it would give us type safety that would avoid these string lookup issues.

Code i fixed was last changed in 2018, context is lost in history 🤷‍♂️
My PR fixes specific bug, but yes we should bring the DNS codebase up to standards (separate from bug fix).