slapos-node: Fix error message

component/slapos/obs.cfg

@@ -29,8 +29,8 @@ download-cache = download-cache
 init +=
   buildout = self.buildout['buildout']
   assert buildout['directory'] == buildout['destdir'] + buildout['rootdir'], (
-    "Buildout MUST BE launched in destdir/rootdir (currently launched in %s but should be launched in %s)",
-    buildout['directory'], buildout['destdir'] + buildout['rootdir'])
+    "Buildout MUST BE launched in destdir/rootdir (currently launched in %s but should be launched in %s)" %
+    (buildout['directory'], buildout['destdir'] + buildout['rootdir']))
 
 [python3-common]
 configure-options +=