[privacy_budget] Fix fieldtrial name in testing config.

"Identifiability" -> "IdentifiabilityStudy" in
`fieldtrial_testing_config.json`.

Bug: 973801
Change-Id: I03e8ca482122973eacd643b46f2a26301f4ed811
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2605354
Auto-Submit: Asanka Herath <asanka@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840201}

testing/variations/fieldtrial_testing_config.json

@@ -3845,7 +3845,7 @@
             ]
         }
     ],
-    "Identifiability": [
+    "IdentifiabilityStudy": [
         {
             "platforms": [
                 "windows",