[Files ranking] Fix fieldtrial testing config.

I made a mistake in our testing config for ramping to beta, this fixes
the experiment name.

Bug: 959679
Change-Id: If5fa7a17c32185a8276e2f40ffd17c2026f43b3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1871717Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708858}

testing/variations/fieldtrial_testing_config.json

@@ -605,7 +605,7 @@
             ],
             "experiments": [
                 {
-                    "name": "AppListZeroStateMixedTypesRanker",
+                    "name": "Enabled_20191017",
                     "enable_features": [
                         "EnableZeroStateMixedTypesRanker"
                     ]