Update create invite expires_at param description

diff --git a/openapi.json b/openapi.json
index a889eb4..7271a8a 100644
--- a/openapi.json
+++ b/openapi.json
@@ -3627,7 +3627,7 @@
                   },
                   "expires_at": {
                     "type": "string",
-                    "default": "controlled by invite_expiry_days site setting"
+                    "description": "optional, if not supplied, the invite_expiry_days site\nsetting is used"
                   }
                 }
               }
diff --git a/openapi.yml b/openapi.yml
index 45791ef..d0efbdd 100644
--- a/openapi.yml
+++ b/openapi.yml
@@ -2579,7 +2579,8 @@ paths:
                   description: optional, either this or `group_id`
                 expires_at:
                   type: string
-                  default: controlled by invite_expiry_days site setting
+                  description: optional, if not supplied, the invite_expiry_days site
+                    setting is used
   "/notifications.json":
     get:
       summary: Get the notifications that belong to the current user

GitHub sha: 7e0f7e0c95234edcf712207721c18719e0cf297c