Update GitHub link in footer content

Author: ralfhandl

_config.yml

@@ -78,4 +78,4 @@ aux_links:
 footer_content: "\xA9 2024 OpenAPI Initiative.
  <br />This work is licensed under the <a rel=\"license\" href=\"https://www.apache.org/licenses/LICENSE-2.0\"\
   >Apache 2.0 License</a>. This site is\
-  \ maintained at <a href=\"https://github.com/OAI/OpenAPI-Specification/tree/gh-pages/\">https://github.com/OAI/OpenAPI-Specification/</a>."
+  \ maintained at <a href=\"https://github.com/OAI/spec.openapis.org/\">https://github.com/OAI/spec.openapis.org/</a>."