docs/api: swagger: fix x-nullable for SystemInfo.Containerd (api v1.46)

This field was added in 812f319a57, but it
looks like redoc doesn't like the field in this location, producing a
warning.

Rendering the docs (`make swagger-docs`) showed a warning:

> Warning: Other properties are defined at the same level as $ref at
> "#/definitions/SystemInfo/properties/Containerd". They are IGNORED
> according to the JsonSchema spec

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/api/v1.46.yaml

@@ -5830,13 +5830,13 @@ definitions:
           - "/var/run/cdi"
       Containerd:
         $ref: "#/definitions/ContainerdInfo"
-        x-nullable: true
 
   ContainerdInfo:
     description: |
       Information for connecting to the containerd instance that is used by the daemon.
       This is included for debugging purposes only.
     type: "object"
+    x-nullable: true
     properties:
       Address:
         description: "The address of the containerd socket."