Passes correct option for custom script to callstats load function.

JitsiConference.js

                 config.callStatsConfIDNamespace
                     || (windowLocation && windowLocation.hostname)
                     || (config.hosts && config.hosts.domain),
-            callStatsCustomScriptUrl: config.callStatsCustomScriptUrl,
+            customScriptUrl: config.callStatsCustomScriptUrl,
             callStatsID: config.callStatsID,
             callStatsSecret: config.callStatsSecret,
             roomName: this.options.name,