content, change overrides to union from object

Technically union is more apt here, because the object will have an
associated schema, it's just unclear what the type for that schema
should be. For instance, does tvdb own the schema under their own
domain? or is it more apt for my.spoor to control it? Going forward I
will use an appropriate tld from the owner of the schema. This should
allow adoption from external providers later if lexicon does take off.

my/spoor/content/external.json

@@ -25,9 +25,9 @@
             }
           },
           "overrides": {
-            "type": "object",
+            "type": "union",
             "description": "User defined overrides for the returned content",
-            "properties": {}
+            "refs": []
           }
         }
       }