Fix the receipts API to be valid swagger

api/client-server/v2_alpha/definitions/error.yaml

@@ -0,0 +1,10 @@
+type: object
+description: A Matrix-level Error
+properties:
+  errcode:
+    type: string
+    description: An error code.
+  error:
+    type: string
+    description: A human-readable error message.
+required: ["errcode"]

api/client-server/v2_alpha/receipts.yaml

@@ -47,6 +47,7 @@ paths:
           required: true
           x-example: "$1924376522eioj:example.com"
         - in: body
+          name: receipt
           description: |-
             Extra receipt information to attach to ``content`` if any. The
             server will automatically set the ``ts`` field.