Add a proper type to the Source field in Mount

This field was missing a type, which meant that the example objects that Swagger tools generate did not work.

Signed-off-by: Wayne Song <wsong@docker.com>

api/swagger.yaml

@@ -231,6 +231,7 @@ definitions:
         type: "string"
       Source:
         description: "Mount source (e.g. a volume name, a host path)."
+        type: "string"
       Type:
         description: |
           The mount type. Available types: