Fix typo, it's branches

Update example about "jobs.branches".

Change-Id: Ie36cfb4b9f53628a3ba1e9619e81171743e4dcd2
diff --git a/doc/source/user/config.rst b/doc/source/user/config.rst
index b2074d1..7234520 100644
--- a/doc/source/user/config.rst
+++ b/doc/source/user/config.rst
@@ -635,7 +635,7 @@
 
          - job:
              name: run-tests
-             branch: stable/2.0
+             branches: stable/2.0
              nodeset: old-release
 
       In some cases, Zuul uses an implied value for the branch