Fix regex for updated_time in PATCH update api test

Change-Id: I2d7a6a3613ed218ca561dc30840e3824d5ce1bd2
Related-Bug: #1721692
diff --git a/api/gabbits/stacks.yaml b/api/gabbits/stacks.yaml
index 39552b0..cb67e71 100644
--- a/api/gabbits/stacks.yaml
+++ b/api/gabbits/stacks.yaml
@@ -140,7 +140,7 @@
     delay: 1.0
   response_json_paths:
     $.stack.stack_status: UPDATE_COMPLETE
-    $.stack.updated_time: /(?!$HISTORY['poll for stack UPDATE_COMPLETE'].$RESPONSE['$.stack.updated_time'])/
+    $.stack.updated_time: /^(?!$HISTORY['poll for stack UPDATE_COMPLETE'].$RESPONSE['$.stack.updated_time'])/
 
 - name: list stack outputs
   GET: $LAST_URL/outputs