docs: invite native speakers to send translation corrections

[silentoplayz]

Summary

The Translate the UI section of the Contributing page now invites native speakers to correct translations in a language Open WebUI already ships, not only to add new languages. Wrong, unnatural or inconsistent labels are easiest to spot for someone who speaks the language. The section already says PRs limited to i18n/localization don't need a maintainer request.

It adds one short paragraph and leaves the rest of the section as it is.

Related issue or discussion

Follows #1457, which was closed. This is the short note in the existing section instead of a new page.

Checklist

• I have reviewed the relevant documentation and matched the existing style.
• This PR meets Open WebUI's contribution standards: it is accurate, relevant to users, narrowly scoped, maintainable, and not promotional content, advertising, lead generation, SEO placement, or a request to list a product, service, provider, integration, gateway, tool, or company primarily for visibility.
• I understand that PRs that do not meet these standards may be closed without review and will not be merged. Repeated, low-quality, off-topic, promotional, or intentionally misleading submissions may result in the contributor being blocked from future participation in Open WebUI repositories.

Notes for reviewers

Only docs/contributing.mdx changes. I did not run npm run build, since the change is one paragraph of plain text with an inline code span.