fix: documentation for /apisix/admin/check_ssl_exists

[frazar]

Please answer these questions before submitting a pull request, or your PR will get closed.

Why submit this pull request?

• [x] Bugfix
• [ ] New feature provided
• [ ] Improve performance
• [ ] Backport patches

What changes will this PR take into?

Fix the documentation for the /apisix/admin/check_ssl_exists endpoint.

Related issues

fix/resolve #2964

Checklist:

• [x] Did you explain what problem does this PR solve? Or what new features have been added?
• [ ] Have you added corresponding test cases?
• [x] Have you modified the corresponding document?
• [x] Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first