update readme link for spec

Author: lightclient

README.md

@@ -78,7 +78,7 @@ $ npm run graphql:validate
 This repository is licensed under [CC0](LICENSE).
 
 
-[playground]: https://playground.open-rpc.org/?schemaUrl=https://raw.githubusercontent.com/ethereum/execution-apis/assembled-spec/openrpc.json&uiSchema[appBar][ui:splitView]=false&uiSchema[appBar][ui:input]=false&uiSchema[appBar][ui:examplesDropdown]=false
+[playground]: https://ethereum.github.io/execution-apis/api-documentation/
 [openrpc]: https://open-rpc.org
 [validator]: https://open-rpc.github.io/schema-utils-js/globals.html#validateopenrpcdocument
 [spellchecker]: https://facelessuser.github.io/pyspelling/