Fieldtrial: rename SyncUssNigori -> SyncUSSNigori

Bug: 922900
Change-Id: Ia9bb93d9bc1206258e9b74fe4fb335bd718f5b1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2070157
Auto-Submit: Maksim Moskvitin <mmoskvitin@google.com>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746978}

testing/variations/fieldtrial_testing_config.json

@@ -5200,7 +5200,7 @@
             ]
         }
     ],
-    "SyncUssNigori": [
+    "SyncUSSNigori": [
         {
             "platforms": [
                 "android",