docs: delete double dashes on ng command in testing guide

[sandervn]

Delete unnecessary double dashes on ng command in testing guide

Closes #30550

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?>

Issue #30550

Does this PR introduce a breaking change?

• Yes
• No

[gkalpak]

Can you, please, remove docs-infra from the commit message (docs-infra refers to changes in the documentation infrastructure; not actual content). E.g.:

docs: delete double dashes on ng command in testing guide

Closes #30550

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     status "ci-codefresh-bazel" is failing

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[brandonroberts]

Caretaker note: The Codefresh build failure is unrelated

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