Fix test_graph

[white-gecko]

Summary of changes

Fix the test_graph test test_guess_format_for_parse_http_text_plain by using a different url to test text/html content-type.

The previous url https://github.com/RDFLib/rdflib/issues/2734 returned 406 Not Acceptable, which made the test fail an was not the intention of the test.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[edmondchuc]

Thanks @white-gecko for the PR. I'm going to close this as it's already fixed in main by #3194.