It's branches, not branch

Change-Id: I6e8a70d122b63a03abef371cb7e8fd9ba6edf99b
diff --git a/zuul/cmd/migrate.py b/zuul/cmd/migrate.py
index b8baf17..af0c256 100644
--- a/zuul/cmd/migrate.py
+++ b/zuul/cmd/migrate.py
@@ -916,7 +916,7 @@
             job_vars.update(self.vars)
 
         if self.branch:
-            output[self.name]['branch'] = self.branch
+            output[self.name]['branches'] = self.branch
 
         if self.files:
             output[self.name]['files'] = self.files