Fix key name in pod-configmap-volume-specific-key.yaml example (#13599)

Changes the key name to `SPECIAL_LEVEL` in the config map volume definition.
This is consistent with the rest of the example. Otherwise, this pod does not
start because there is no key with that name in the `special-config` config map.

content/en/examples/pods/pod-configmap-volume-specific-key.yaml

@@ -15,6 +15,6 @@ spec:
       configMap:
         name: special-config
         items:
-        - key: special.level
+        - key: SPECIAL_LEVEL
           path: keys
   restartPolicy: Never