docs: show --path option on build help

[janoshrubos]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
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What is the current behavior?

Result of tns build android/ios -h command does not include --path option.

What is the new behavior?

Show --path option in result of tns build android/ios -h command.

Fixes #5339