Update name of ios lookalike testing config

Modifies name for consistency with other relevant configs.

Change-Id: I48677a0283fcd92e2147ab0492f2982d75c59ec1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432134Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Livvie Lin <livvielin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811466}

testing/variations/fieldtrial_testing_config.json

@@ -3389,7 +3389,7 @@
             ]
         }
     ],
-    "IOSLookalikeUrlInterstitial": [
+    "IOSLookalikeInterstitials": [
         {
             "platforms": [
                 "ios"