Adapt SNAPSHOT plugin api example to the 3.1 version

Change-Id: I9d3b89d790ea6cd573d33c493d4224468c17fa82

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 473f424..2eebcd6 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -16,4 +16,4 @@
 gerrit_api()
 
 # Load snapshot Plugin API
-#gerrit_api(version = "3.0.xy-SNAPSHOT")
+#gerrit_api(version = "3.1.xy-SNAPSHOT")