Fixed Typo in FieldTrial Testing Config ExploreSitesDense Study

This is triggering a Finch warning when trying to make changes to the
GCL file. (cl/275848776)

Change-Id: Id95ed8dd6e34ab48f171ec8e3ec8614567ea0519
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879867Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Sebastien Lalancette <seblalancette@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Auto-Submit: Sebastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709454}

testing/variations/fieldtrial_testing_config.json

@@ -2458,7 +2458,7 @@
                     "params": {
                         "denseVariation": "titleBottom",
                         "exp": "games-top",
-                        "mostLikelyvariation": "groupedIcon",
+                        "mostLikelyVariation": "groupedIcon",
                         "variation": "mostLikelyTile"
                     },
                     "enable_features": [