update web hook doc to include trailing slash

When using a URL without a trailing slash GitHub reports a http status code 302 from Jenkins and no trigger message appears in the Jenkins log.

With a trailing slash the web hook reports a status 200 and the Jenkins log shows the trigger event, e.g.:

    <timestamp> INFO org.jenkinsci.plugins.ghprb.GhprbRootAction doIndex
    Got payload event: issue_comment

Since the trailing slash seems to be crucial the tool tip should actually include it.