grammatical error in alert.js (#32062)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

examples/cms-strapi/components/alert.js

@@ -14,7 +14,7 @@ export default function Alert({ preview }) {
         <div className="py-2 text-center text-sm">
           {preview ? (
             <>
-              This is page is a preview.{' '}
+              This page is a preview.{' '}
               <a
                 href="/api/exit-preview"
                 className="underline hover:text-cyan duration-200 transition-colors"