Fix error on warning msg (#4279)

lib/plugins/compare/index.js

@@ -129,7 +129,7 @@ export default class ComparePlugin extends SitespeedioPlugin {
             )
               log.warning(
                 'The baseline test has %s runs and you current have %s. You should make sure you test the same amount of runs',
-                baseline.timestamps.length,
+                baseline.browsertime.timestamps.length,
                 this.options.browsertime.iterations
               );
             log.info('Got a baseline:' + id + '-' + this.page);