openapi: Fix examples in response for deactivating user endpoints.

This commit fixes examples in "400" response for deactivating user
endpoints to have msg as "Cannot deactivate the last organization
owner" instead of "Cannot deactivate the last organization
administrator".

We had already removed the restriction on deactivating last admin
and added it for last owner, while adding owner role.

templates/zerver/api/deactivate-user.md

@@ -28,6 +28,6 @@ A typical successful JSON response may look like:
 {generate_code_example|/users/{user_id}:delete|fixture(200)}
 
 An example JSON error response when attempting to deactivate the only
-organization administrator:
+organization owner in an organization:
 
 {generate_code_example|/users/{user_id}:delete|fixture(400)}

zerver/openapi/zulip.yaml

@@ -4499,7 +4499,7 @@ paths:
                   - $ref: "#/components/schemas/JsonError"
                   - example:
                       {
-                        "msg": "Cannot deactivate the only organization administrator",
+                        "msg": "Cannot deactivate the only organization owner",
                         "result": "error",
                       }
   /users/me/{stream_id}/topics:
@@ -5597,7 +5597,7 @@ paths:
                   - $ref: "#/components/schemas/JsonError"
                   - example:
                       {
-                        "msg": "Cannot deactivate the only organization administrator",
+                        "msg": "Cannot deactivate the only organization owner",
                         "result": "error",
                       }
   /realm/filters: