[Synthetics UI] Update copy for test now functionality on private locations (#137912)

* Update copy for test now functionality on private locations.

* Update copy based on PR feedback.

Author: justinkambic

x-pack/plugins/synthetics/public/legacy_uptime/components/overview/monitor_list/columns/test_now_col.tsx

@@ -82,7 +82,7 @@ export const TEST_NOW_AVAILABLE_LABEL = i18n.translate(
 export const PRIVATE_AVAILABLE_LABEL = i18n.translate(
   'xpack.synthetics.monitorList.testNow.available.private',
   {
-    defaultMessage: 'For now, Test now is disabled for private locations monitors.',
+    defaultMessage: `You can't currently test monitors running on private locations on demand.`,
   }
 );