SDK-696 fix test for mutability change. Missed this during my review of the change.

Author: echo-branch

Branch-SDK-Tests/BranchEvent.Test.m

@@ -271,8 +271,9 @@ - (void) testExampleSyntax {
 
     BranchEvent *event = [BranchEvent standardEvent:BranchStandardEventCompleteRegistration];
     event.eventDescription = @"Product Search";
-    event.searchQuery = @"product name";
-    event.customData[@"rating"] = @"5";
+    event.searchQuery = @"product name";    
+    event.customData = @{ @"rating": @"5" };
+    
     [event logEvent];
 }