[iOS] Fix Typo in FieldTrial Testing Config for Full Page Screenshots

Bug: 1103778
Change-Id: Ia10232a219862fce9b79c08a383a8dabd307e438
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2460742
Commit-Queue: Sebastien Lalancette <seblalancette@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Sebastien Lalancette <seblalancette@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815273}

testing/variations/fieldtrial_testing_config.json

@@ -3357,7 +3357,7 @@
             ]
         }
     ],
-    "IOSFullPageScreenshots": [
+    "IOSFullPageScreenshot": [
         {
             "platforms": [
                 "ios"