README: Test build with stable branch setting

README.md

@@ -263,13 +263,13 @@ by the hydra server.  If significant changes have been made, these can
 be manually tested before pushing
 
 ```bash
-nix-build release.nix --attr tested
+nix-build release.nix --arg stableBranch true --attr tested
 ```
 
 A build of the channel tarball can also be done
 
 ```bash
-nix-build release.nix --attr channel
+nix-build release.nix --arg stableBranch true --attr channel
 ```
 
 The resulting channel tarball can even be imported as new channel