openapi: Fix principals array type.

We do not accept heterogeneous arrays containing both user ids and
email addresses.

This also happens to disallow an empty array, which is fine since the
principals parameter should be omitted if the default to the calling
user is desired.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/openapi/zulip.yaml

@@ -15431,11 +15431,13 @@ components:
       content:
         application/json:
           schema:
-            type: array
-            items:
-              oneOf:
-                - type: string
-                - type: integer
+            oneOf:
+              - type: array
+                items:
+                  type: string
+              - type: array
+                items:
+                  type: integer
           example: ["ZOE@zulip.com"]
     ReactionType:
       name: reaction_type