chore(help): move from tns to ns in help

[jcassidyav]

Also from tns-android or tns-ios to @nativescript/android or @nativescript/ios.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

Content displayed by help is in the form tns <command>.

What is the new behavior?

Changes help content to ns <command>

Also some minor grammar and changing the references in help from tns-android/io to @NativeScript/ns.