Fix synthetics docs for read-only user (#4950)

Author: mdbirnstiehl

docs/en/observability/synthetics-role-read.asciidoc

@@ -39,7 +39,7 @@ and grant the following privileges:
 | Read-only access to synthetics alert indices.
 
 | {kibana-ref}/kibana-privileges.html[Kibana]
-| `Synthetics and Uptime`: `All`
+| `Synthetics and Uptime`: `read`
 | Access to the {synthetics-app} in {kib}.
 
 |====