docs: fix referrer typedef in OnCompletedDetails (#16661)

docs/api/web-request.md

@@ -200,6 +200,7 @@ redirect is about to occur.
     * `method` String
     * `webContentsId` Integer (optional)
     * `resourceType` String
+    * `referrer` String
     * `timestamp` Double
     * `responseHeaders` Object
     * `fromCache` Boolean