Fix text alignment on mobile devices for zha-add-devices-page.ts (#19517)

discovery_text ("Make sure your devices are in pairing mode. Check the instructions of your device on how to do this.") is showing left aligned on mobile, which is inconsistent with the discovered_text ("Devices will show up here once discovered.") underneath.

src/panels/config/integrations/integration-panels/zha/zha-add-devices-page.ts

@@ -218,6 +218,7 @@ class ZHAAddDevicesPage extends LitElement {
           display: flex;
           flex-direction: column;
           align-items: center;
+          text-align: center;
         }
         .content {
           display: flex;