fix: rename override -> override_existing (#151)

"override" is the incorrect variable name when using UpsertUsersAsync api. Instead "override_existing" should be used

Author: sinapy

src/UsersBatch.cs

@@ -22,7 +22,7 @@ public async Task<IEnumerable<User>> UpsertUsersAsync(IEnumerable<User> users, b
             var body = new Dictionary<string, object>
             {
                 { "users", users },
-                { "override", overrideExisting },
+                { "override_existing", overrideExisting },
             };
             var request = _client.BuildAppRequest("users/", HttpMethod.Post);
             request.SetJsonBody(StreamJsonConverter.SerializeObject(body));