v4 Migration docs still refer to .has-danger

Created by: exige81

From docs/4.0/migration.md

Moved element resets to the _reboot.scss file.
Renamed .control-label to .form-control-label.
Renamed .input-lg and .input-sm to .form-control-lg and .form-control-sm, respectively.
Dropped .form-group-* classes for simplicity's sake. Use .form-control-* classes instead now.
Dropped .help-block and replaced it with .form-text for block-level help text. For inline help text and other flexible options, use utility classes like .text-muted.
Dropped .radio-inline and .checkbox-inline.
Horizontal forms overhauled:
- Dropped the .form-horizontal class requirement.
- .form-group no longer applies styles from the .row via mixin, so .row is now required for horizontal grid layouts (e.g., <div class="form-group row">).
- Added new .form-control-label class to vertically center labels with .form-controls.
- Added new .form-row for compact form layouts with the grid classes (swap your .row for a .form-row and go).
Added custom forms support (for checkboxes, radios, selects, and file inputs).
Added HTML5 form validation support via CSS's :invalid and :valid pseudo-classes. - Renamed .has-error to .has-danger.
Renamed .form-control-static to .form-control-plaintext.

This was replaced by .is-invalid