Fix typo in swagger.yaml for NetworkPrune operation

Signed-off-by: Joffrey F <joffrey@docker.com>
(cherry picked from commit 20c5fa37585b76fe41cb98163bc0b9df2a3e2ec3)
Signed-off-by: Victor Vieux <victorvieux@gmail.com>

api/swagger.yaml

@@ -6306,7 +6306,7 @@ paths:
           schema:
             type: "object"
             properties:
-              VolumesDeleted:
+              NetworksDeleted:
                 description: "Networks that were deleted"
                 type: "array"
                 items: