Use registration key instead of machine key

docs/ref/api.md

@@ -54,7 +54,7 @@ Headscale server at `/swagger` for details.
 
     ```console
     curl -H "Authorization: Bearer <API_KEY>" \
-        -d user=<USER> -d key=<KEY> \
+        -d user=<USER> -d key=<REGISTRATION_KEY> \
         https://headscale.example.com/api/v1/node/register
     ```