Update the synthetics README file to add the missing config for the script. (elastic#235756)

## Summary

Update the synthetics README file to add the missing config for the
script.

Author: fkanout

x-pack/packages/kbn-synthetics-private-location/README.md

@@ -53,4 +53,9 @@ xpack.fleet.outputs:
     is_default: true
     is_default_monitoring: true
     hosts: ['http://host.docker.internal:9200'] # For enrolling dockerized agents
+
+xpack.fleet.packages:
+  - name: fleet_server
+    version: latest
+
 ```