Update RTD schema to use 'main'

The main branch for the RTD repo was renamed from 'master'.

Author: sirosen

src/check_jsonschema/catalog.py

@@ -182,7 +182,7 @@ def _githubusercontent_url(owner: str, repo: str, ref: str, path: str) -> str:
         "url": _githubusercontent_url(
             "readthedocs",
             "readthedocs.org",
-            "master",
+            "main",
             "readthedocs/rtd_tests/fixtures/spec/v2/schema.json",
         ),
         "hook_config": {