Fix site_url for readthedocs 404 (#66)

Author: maxrjones

mkdocs.yml

@@ -4,7 +4,7 @@ repo_name: virtual-zarr/obspec-utils
 repo_url: https://github.com/virtual-zarr/obspec-utils
 site_description: Utilities for interacting with object storage, based on obspec.
 site_author: Max Jones
-site_url: https://obspec-utils.readthedocs.io
+site_url: !ENV [READTHEDOCS_CANONICAL_URL, 'https://obspec-utils.readthedocs.io']
 docs_dir: docs
 
 extra: