docs: Use full Gitea URL in usage examples

Short-form action references default to GitHub.
Full URL required for Gitea-to-Gitea action references.

README.md

@@ -22,14 +22,16 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: johno/gitea-actions/nix-setup@main
+      - uses: https://git.johnogle.info/johno/gitea-actions/nix-setup@main
       - run: nix flake check
 ```
 
+> **Note:** Full URL required for Gitea Actions to reference actions from the same instance (short-form defaults to GitHub).
+
 **With custom cache prefix:**
 
 ```yaml
-- uses: johno/gitea-actions/nix-setup@main
+- uses: https://git.johnogle.info/johno/gitea-actions/nix-setup@main
   with:
     cache-name: 'my-project-'
 ```