fix typo in conferences api field whitelist

fixes CNVS-14298

test plan
- make a conference with advanced settings
- get the conference index using the api
- ensure that the has_advanced_settings field is true

Change-Id: Iee5c10ea4d346d86ad06e8040a5b2d7c3cf2bb52
Reviewed-on: https://gerrit.instructure.com/37754
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>