In cat.snapshots, repository is required

Closes #17216

rest-api-spec/src/main/resources/rest-api-spec/api/cat.snapshots.json

@@ -8,6 +8,7 @@
       "parts": {
         "repository": {
           "type" : "list",
+          "required": true,
           "description": "Name of repository from which to fetch the snapshot information"
         }
       },