Attempt to fix type error

The error:

```
Error: Argument of type "dict[str, int]" cannot be assigned to parameter "value" of type "str" in function "__setitem__"
  "dict[str, int]" is not assignable to "str" (reportArgumentType)
```

Author: chigby

wagtail_newsletter/campaign_backends/mailchimp.py

@@ -203,15 +203,16 @@ def get_campaign_request_body(
         }
 
         if recipients is not None:
-            body["recipients"] = {
+            recipients_mapping: dict[str, Any] = {
                 "list_id": recipients.audience,
             }
 
             if recipients.segment:
                 segment_id = int(recipients.segment.split("/")[1])
-                body["recipients"]["segment_opts"] = {
+                recipients_mapping["segment_opts"] = {
                     "saved_segment_id": segment_id,
                 }
+            body["recipients"] = recipients_mapping
 
         return body