Fixed typo in discovery.py

Replaced "additonalProperties" with "additionalProperties"

Change-Id: I550f0d49bf1a55b6b4b02a0c3a449725c25f63e3

Author: Mridula Joshi

glance/api/v2/discovery.py

@@ -199,7 +199,7 @@ def get_usage(self, response, usage):
             'additionalProperties': True,
             'validation_data': {
                 'type': 'object',
-                'additonalProperties': False,
+                'additionalProperties': False,
                 'properties': {
                     'usage': {'type': 'integer'},
                     'limit': {'type': 'integer'},