[Docs] Name remote cluster "two" (#42976)

All the text and the search example refer to the remote cluster as
"two" rather than "cluster_two". This commit changes the definition
that is stored through the settings API to match the rest of the
examples.

x-pack/docs/en/security/ccs-clients-integrations/cross-cluster.asciidoc

@@ -52,10 +52,10 @@ PUT _cluster/settings
   "persistent": {
     "cluster": {
       "remote": {
-        "cluster_one": {
+        "one": {
           "seeds": [ "10.0.1.1:9300" ]
         },
-        "cluster_two": {
+        "two": {
           "seeds": [ "10.0.2.1:9300" ]
         }
       }
@@ -83,7 +83,7 @@ First, enable cluster `one` to perform cross cluster search on remote cluster
 PUT _cluster/settings
 {
   "persistent": {
-    "cluster.remote.cluster_two.seeds": [ "10.0.2.1:9300" ]
+    "cluster.remote.two.seeds": [ "10.0.2.1:9300" ]
   }
 }
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