document the default for /servers (#2861)

https://spec.openapis.org/oas/v3.1.0#fixed-fields

schemas/v3.1/schema.json

@@ -20,7 +20,10 @@
       "type": "array",
       "items": {
         "$ref": "#/$defs/server"
-      }
+      },
+      "default": [
+        { "url": "/" }
+      ]
     },
     "paths": {
       "$ref": "#/$defs/paths"

schemas/v3.1/schema.yaml

@@ -17,6 +17,8 @@ properties:
     type: array
     items:
       $ref: '#/$defs/server'
+    default:
+      - url: /
   paths:
     $ref: '#/$defs/paths'
   webhooks: