prober: Fix the url to fiddle server.
prober: Fix the url to fiddle server.

According to compute_engine_scripts/webtry/vm_config.sh the instance
name is skia-webtry.

BUG=None
R=borenet@google.com,jcgregorio@google.com

Review URL: https://codereview.chromium.org/1249383002

google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot