fix: oauth device flow scopes

Correct the name of variable from scopes to scope in the OAuth device
flow implementation as documented in the GitHub API docs.
https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/authorizing-oauth-apps#step-1-app-requests-the-device-and-user-verification-codes-from-github

Author: stevenh

githubkit/auth/oauth.py

@@ -36,7 +36,7 @@ def create_device_code(
     url = base_url.copy_with(raw_path=base_url.raw_path + b"login/device/code")
     body = {"client_id": client_id}
     if scopes:
-        body["scopes"] = " ".join(scopes)
+        body["scope"] = " ".join(scopes)
     response = yield httpx.Request(
         "POST",
         url,