[doc]: Help text documentation is inaccurate

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### Description of issue

The doc at https://opensource.adobe.com/spectrum-web-components/components/help-text/#negative is incorrect.

It appears that the slot for the negative help text should be `negative-help-text` as opposed to `help-text-negative` and the example is not really showing the negative help text as it stands. Manually changing it in the inspector seems to show show the expect color and text _Passwords must be at least 8 characters_

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Open the Help Text documentation at the provided URL and inspect the #negative section. Verify the negative help-text slot name and the example against the expected color and text, “Passwords must be at least 8 characters.” Done means the documentation uses the correct slot and demonstrates negative help text accurately.

Written by the indexing model from the issue text.