Fix MarketoAPI::Campaigns

We used to get exceptions similar to the following because the SOAP
operation was improperly specified.
---
Unable to find SOAP operation: :RequestCampaign Operations provided by
your service: [:delete_m_objects, :describe_m_object, :get_m_objects,
:sync_m_objects, :get_campaigns_for_source, :get_import_to_list_status,
:get_lead, :get_lead_activity, :get_lead_changes, :get_multiple_leads,
:import_to_list, :list_m_objects, :list_operation, :request_campaign,
:schedule_campaign, :sync_lead, :sync_multiple_leads,
:sync_custom_objects, :delete_custom_objects, :get_custom_objects,
:merge_leads, :get_channels, :get_tags]