Correct the example of zuul enqueue command

Change-Id: I9d73515d796f591c4c5984834959d5625d1c61b9
diff --git a/doc/source/client.rst b/doc/source/client.rst
index f0b784a..1a843e5 100644
--- a/doc/source/client.rst
+++ b/doc/source/client.rst
@@ -28,4 +28,6 @@
 
 Example::
 
-  zuul enqueue --trigger gerrit --pipeline check --project example_project --change 12345 --patchset 1
+  zuul enqueue --trigger gerrit --pipeline check --project example_project --change 12345,1
+
+Note that the format of change id is <number>,<patchset>.