Use cron in pollscm

Fixes this warning

WARNING:jenkins_jobs.modules.triggers:Your pollscm usage is deprecated,
please use the syntax described in the documentation instead

Also it is recommended by the docs
https://docs.openstack.org/infra/jenkins-job-builder/triggers.html#cron

And is done every where else in ci
https://github.com/GerritCodeReview/gerrit-ci-scripts/search?utf8=✓&q=pollscm+&type=

Change-Id: I534469e2d54da7f2b4908dbc6cc87d2ee4d717c1
diff --git a/jenkins/gitiles-webapp.yaml b/jenkins/gitiles-webapp.yaml
index 1ddccbf..d418629 100644
--- a/jenkins/gitiles-webapp.yaml
+++ b/jenkins/gitiles-webapp.yaml
@@ -26,7 +26,8 @@
          skip-tag: true
 
     triggers:
-      - pollscm: 'H/50 * * * *'
+      - pollscm:
+          cron: 'H/50 * * * *'
 
     builders:
       - shell: |